Russia on Monday warned US lawmakers debating a landmark nuclear arms control treaty that any changes would kill the accord, which faced a critical US Senate test vote on the uncertain path to approval.

US President Barack Obama, who signed the new Strategic Arms Reduction Treaty (START) with Russian President Dmitry Medvedev in April, was courting swing-vote senators by telephone, the White House said.

Obama and Vice President Joe Biden "continue to communicate with senators in order to ensure they have the most updated information," said spokesman Robert Gibbs, who predicted the agreement would win ratification.

Russian Foreign Minister Sergei Lavrov warned US senators that START "completely meets the national interests of both Russia and the United States" and "cannot be reopened, becoming the subject of new negotiations."

His comments to the Interfax news agency came as angry Republicans, lacking the votes to amend the treaty, accused Democrats of rushing the agreement through to hand Obama a major year-end diplomatic victory.

"No senator should be forced to make decisions like this so we can tick off another item on someone's political checklist before the end of the year," said Republican Senate Minority Leader Mitch McConnell.

After five days of arguing in public about amending the pact -- sending it back to the negotiating table, effectively killing it -- US Senators met behind closed doors to discuss classified intelligence matters tied to the accord.

They were expected to discuss issues including the verification and monitoring regimes designed to ensure the two sides do not cheat on the agreement as well as the upkeep of the US nuclear arsenal.

After their secret debate, lawmakers were expected to vote on -- and defeat -- more Republican treaty amendments calling notably for more inspections under the accord and for talks on restricting Russian tactical nuclear weapons.

And the Senate was to vote Tuesday on ending debate on the treaty and moving towards ratification, which requires 67 votes if all 100 senators are present.

Democrats have expressed optimism about rallying enough Republicans to approve the treaty, pointing to a 66-32 vote to begin debate. One lawmaker who supports START was absent.

The agreement -- which has the support of virtually every present and past US foreign policy or national security heavyweight -- restricts each nation to a maximum of 1,550 deployed warheads, a cut of about 30 percent from a limit set in 2002, and 800 launchers and bombers.

The accord would also return US inspectors who have been unable to monitor Russia's arsenal since the treaty's predecessor lapsed in December 2009.

Some US lawmakers, seeking a face-saving way to express concerns about the treaty without killing it, were quietly discussing possible amendments to the US Senate's resolution of ratification -- which would not force new negotiations.

Obama has made approving the treaty a key part of his efforts to "reset" relations with Russia.

And the White House has pushed for a vote this year, when it needs nine Republicans to win ratification, rather than next year, when it will need 14 in the new Congress.

Lavrov encouraged Washington to move quickly but appeared to put no time limit on a final vote.

"We would prefer not to think about the negative consequences" of this treaty not being passed," Russia's top diplomat told the agency.

"We expect the ratification process to be completed in near future."