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

"This is too important to delay," said 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.

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."

Obama has called on the Senate to meet that timetable, but Democrats and their two independent allies hold only 59 of the 67 votes needed to approve the treaty, 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.

"You could probably quibble over renaming a post office on any given day in the United States Senate. That's not to say at the end of the day there isn't enough space and time to do this this year," Obama spokesman Robert Gibbs said in Prague, where Obama and Russian President Dmitry Medvedev signed the deal.

The White House and its allies have underlined that most past arms control deals have easily cleared the US Senate, which is charged by the US Constitution with voting 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, who did not give a target date for such a vote, said he would work with Kerry, Senate Armed Services Committee Chairman Carl Levin, and Senate Intelligence Committee Chairwoman Dianne Feinstein, to approve the treaty.

Republicans have 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 caution they will oppose the pact if they think it will hamper US missile defense plans bitterly opposed by Russia.

Senate Republicans, led by Minority Leader Mitch McConnell, have also said they want assurances that the Obama 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.