US President Barack Obama and his Russian counterpart Dmitry Medvedev have finalized a historic new deal to cut long-range nuclear arms, argeeing to slash the number of deployed warheads by a third.

After months of intense negotiations, the pair sealed what Obama called "the most comprehensive arms control agreement in nearly two decades," as they hailed improved ties that hit a low under US president George W. Bush.

The new pact, due to be signed April 8 in Prague by both the presidents of the former Cold War foes, replaces the landmark 1991 Strategic Arms Reduction Treaty (START), which expired in December.

In the Czech capital last year, Obama unveiled a plan to purge the world of atomic weapons by cutting stockpiles, curtailing testing, choking fissile production and securing loose nuclear material.

Obama -- standing Friday next to Secretary of State Hillary Clinton, Defense Secretary Robert Gates and other key officials -- stressed that the new START treaty was a "fundamental part of that effort" for a nuclear-free world.

He said the deal also advances his priority to "reset" ties between the two countries, which have deepened cooperation on the US-led war against Muslim extremists in Afghanistan and on efforts to curb Iran's nuclear ambitions.

In Moscow, the Kremlin said "the presidents agreed that the new treaty marks the transfer of Russian-US cooperation to a higher level in the development of new strategic ties."

The Union of Concerned Scientists (UCS) warmly welcomed the treaty.

"This treaty will send a powerful, unambiguous message to the rest of the world that the United States and Russia are serious about reducing the nuclear threat," the UCS's Sean Meyer said in a statement.

"And the timing couldn't be better, given President Obama is hosting an international nuclear security summit in April and the Nuclear Non-Proliferation Treaty review conference in May," he said.

Obama said the treaty "significantly reduces" US and Russian missiles and launchers, and sets up a "strong and effective" verification regime.

It specifies limits of 1,550 deployed warheads, which is about 30 percent lower than a previous upper warhead limit set in 2002.

The UCS said the new treaty counts each deployed missile warhead as one, and each deployed bomber as one warhead, and under these rules, currently the United States has 1,762 warheads and Russia has 1,741.

"The treaty does not cover the thousands of warheads both countries have in storage," it said in a statement.

The treaty limits missile forces to 800 deployed and non-deployed intercontinental ballistic missile launchers, submarine launched ballistic missile launchers, and heavy bombers equipped for nuclear weapons.

The cap on deployed intercontinental ballistic missiles and submarine launched missiles is set at 700, the White House said.

Clinton, who traveled to Moscow last week in a bid to clear the last hurdles to the deal, said she was "confident" the Obama administration would be able to convince the Senate to ratify the treaty.

A senior State Department official said the treaty will be "submitted for ratification in late spring."

Obama's Senate ally John Kerry, who chairs the chamber's foreign relations committee, urged Republicans to end the feuding.

"We can't squander this opportunity to reset both our relations with Russia and our role as the world leader on nuclear nonproliferation," he said.

Republican senators, who could deny the administration the two-thirds majority needed for ratification, vowed to closely examine the document, although key Republican Richard Lugar appeared to back the deal.

Republicans have warned they will not support any deal that harms US missile defense systems, and have said they want a detailed plan for modernizing the US nuclear weapons stockpile before taking up the new treaty.

But Gates said the treaty does not set constraints on US plans to develop and improve missile defense systems.

Russian Foreign Minister Sergei Lavrov meanwhile made it clear that Moscow could back out if the United States went too far with its plans to counter missile attacks by setting up defenses in eastern Europe.

Moscow has strongly opposed US missile defense plans and has long insisted on an explicit recognition of the link between offensive and defensive systems in any new strategic arms pact.

Last year, Obama canceled plans to build missile bases and radar installations in the Czech Republic and Poland, saying ship-based missile interceptors could protect against Iran and North Korea.

Provided the new treaty is ratified, the agreement will hand Obama his second major political victory in a week after he signed his landmark health care overhaul into law on Tuesday.

Signed in 1991, START led to huge reductions in the Russian and US nuclear arsenals and imposed verification measures to build trust between the two former Cold War foes.

US senators eager to review new US-Russia nuclear pact
Washington (AFP) March 26, 2010 - President Barack Obama's Democratic Senate allies on Friday quickly embraced a new US-Russia nuclear arms reduction treaty, as his Republican critics vowed to closely examine the accord.

Democratic Senator John Kerry, who chairs the chamber's foreign relations committee, urged his colleagues to set aside bitter election-year feuding and ratify the pact in 2010.

"This is a moment for statesmanship," said Kerry. "We can't squander this opportunity to reset both our relations with Russia and our role as the world leader on nuclear nonproliferation."

Kerry, whose panel would take up the accord before any full Senate vote, said earlier this week that he would hold hearings as soon as possible on the pact to replace the 1991 START treaty that expired in December.

"I know there has been a partisan breakdown in recent years, but we can renew the Senate's bipartisan tradition on arms control and approve ratification of this new treaty in 2010. I know that can happen," said Kerry.

"Let's get it done," the former presidential hopeful said.

The US Constitution tasks the Senate with ratifying treaties, which requires a two-thirds majority in the 100-seat body.

Democrats and their two independent allies control 59 votes, Republicans hold 41.

In the past, Republicans have warned they will not support any deal that harms US missile defense systems that have angered Russia, and have said they want a detailed plan for modernizing the US nuclear weapons stockpile before taking up the new treaty.

The top Republican on Kerry's committee, Senator Richard Lugar, praised the "dedicated effort" of US and Russian diplomats and said he would work with Kerry "so that we can work quickly to achieve ratification of the new treaty."

Lugar, a champion of nuclear arms control efforts for decades and a co-author of the Nunn-Lugar program that keeps close tabs on Soviet-era nuclear stockpiles, is an influential voice in his party on foreign affairs.

Obama's Republican rival in the 2008 White House race, Senator John McCain, "will carefully and thoroughly review the policy before arriving at a conclusion," said his spokeswoman, Brooke Buchanan.

Senator John Thune, the number four Republican in the Senate, said he had "concerns" about the "deep cuts" the treaty would enact and that they would require Washington to upgrade its existing weapons systems.

"This is a critical issue to US national security and will take time for the Senate to review," said Thune.

But Democratic Senate Majority Leader Harry Reid said such treaties have historically "received broad bipartisan support, and I am confident that this treaty will achieve similar support from both sides of the aisle."

Apparently anticipating Republican objections, Reid said in a statement that the new pact "comfortably maintains America's nuclear capabilities so that we can keep America secure and deter our adversaries."

And he also praised the administration for reaching a deal "without constraining America's missile defense system in any way" and praised plans to invest five billion dollars over five years "to ensure that our nuclear deterrent remains safe, secure and effective."

Britain hails Russia-US nuke deal, eyes own cuts
London (AFP) March 26, 2010 - Britain hailed Friday a Russia-US deal to cut long-range nuclear arms as "an important further step" towards a nuclear-free world -- and vowed to include its own atomic arsenal in future negotiations.

"Today's historic announcement will help pave the way for further reductions," said Foreign Secretary David Miliband of the agreement between Washington and Moscow to slash the number of deployed warheads by a third.

"As the (British) prime minister (Gordon Brown) told (US) President Barack Obama when they spoke yesterday, the UK welcomes this agreement which is an important further step towards a world free from nuclear weapons," he added.

In simultaneous announcements in Washington and Moscow, Obama and Russian President Dmitry Medvedev said the deal raised cooperation between the two former Cold War foes to a new level.

The British minister welcomed the "landmark agreement" on a successor treaty to the Strategic Arms Reduction Treaty (START), which expired in December.

"The international community must now seize the opportunity this creates for the 2010 NPT (Nuclear Non-Proliferation Treaty) review conference and beyond," he said, referring to an international summit on nuclear security in Washington on April 12-13.

Miliband added: "The UK is firmly committed to working towards a world free from nuclear weapons. We have already taken a number of steps to ensure we retain only the absolute minimum capability required to provide effective deterrence."

"We will continue to keep the number of warheads under review and make further reductions wherever possible," he said.

"As soon as it becomes useful to do so, the UK stands ready to include our nuclear arsenal in a future multilateral disarmament process," he added.

Prime Minister Gordon Brown said last December that Britain is prepared to reduct its fleet of nuclear-capable submarines from four to three, although without proposing changing the number of nuclear warheads.

Britain is thought to have the world's fifth biggest nuclear weapons inventory, according to figures last year from the Bulletin of the Atomic Scientists.