US President Barack Obama and his top Democratic allies called Thursday for quick Senate ratification of a US-Russia treaty committing the former Cold War foes to major nuclear arms cuts.

Obama wasted no time in pushing for ratification in an interview with ABC News targeting a domestic political audience as skeptical Republicans expressed concerns about the new agreement and gave no sign of endorsing his call for a final vote this year.

"When they have had the opportunity to fully evaluate this treaty, they will come to the conclusion that this is in the best interest of the United States," Obama told ABC about Republican senators.

"I will also say to those in the Senate who have questions... that this is absolutely vital for us to deal with the broader issues of nuclear proliferation that are probably the number one threat that we face in the future."

Democratic Senator John Kerry, who chairs the Senate Foreign Relations Committee tasked with taking up the pact before a full vote by the entire chamber, said it was "too important to delay."

Kerry said he would work with the panel's top Republican, Richard Lugar, after the Obama administration submits the full treaty "in early May" to hold hearings and "see that this historic treaty is ratified this year."

Earlier in Prague, where he signed the treaty with Russian President Dmitry Medvedev, Obama said he was "actually quite confident that Democrats and Republicans in the United States Senate, having reviewed this, will see that the United States has preserved its core national security interests."

But Democrats and their two independent allies hold only 59 of the 67 votes needed to approve the deal, meaning they will need to rally eight Republicans.

With November mid-term elections on the horizon and a bitterly partisan climate in Washington, prospects for doing so were unclear, and Republicans have signaled strong concerns over the new accord.

The White House and its allies have underlined that most past arms control deals have easily cleared the Senate, which has a constitutional duty to vote to bring such agreements into force.

"The Senate has a long history of approving strategic arms control treaties by overwhelming margins and I am confident we will renew that spirit of cooperation and bipartisan tradition," said Kerry.

Democratic Senate Majority Leader Harry Reid praised Obama's signing of the "historic treaty" and -- taking aim at a key Republican objection -- said US missile defense plans are "in no way constrained."

"I am confident that this agreement will receive the 67 votes from both sides of the aisle needed for passage. There is no need to play politics with something as important as this is to our national security," he said.

Reid did not give a target date for a vote.

Republicans stopped well short of vowing to defeat the new pact.

But they warned that Obama must submit a comprehensive plan for upgrading US nuclear laboratories and modernizing the US nuclear arsenal before the Senate takes up the treaty, and cautioned they would oppose it if it hampers US missile defense plans bitterly opposed by Russia.

Obama, who said the treaty was "only a start," told ABC that "we were firm, and are now absolutely confident that this in no way impedes our ability to move forward on the missile defense program, that's designed not to target Russia."

Senate Republicans, led by Minority Leader Mitch McConnell, have also said they want assurances the administration will preserve the so-called "triad" of land-, sea- and air-based nuclear weapons.

"The Senate will assess whether or not the agreement is verifiable, whether it reduces our nation's ability to defend itself and our allies from the threat of nuclear armed missiles, and whether or not this administration is committed to preserving our own nuclear triad," said McConnell.

Republican Senators John McCain and Jon Kyl, both of Arizona, said in a joint statement that they were concerned about language they described as having "the potential to constrain improvements to US missile defenses, if objected to by the Russians."