A motion to limit further debate on the treaty passed with 67 votes, the same number needed to secure Senate approval. A final vote on the accord was set for Wednesday after lawmakers debate a rash of last-minute amendments.

"This treaty will make America safer and restore our leadership in global efforts to stop nuclear proliferation," Senate Democratic Leader Harry Reid said.

But Senator John Kerry, who led the floor debate as chairman of the Foreign Relations Committee, cautioned: "This is not over. We have to count every vote."

At least 12 Republicans have said that they will vote with Democrats to approve the pact, which would give Obama his third major victory on Capitol Hill in less than a week.

He earlier won repeal of the US ban on gays serving openly in the military and passage of an US$858 billion deal with Republicans to extend expiring tax cuts and spur economic growth.

The treaty, which would cut strategic atomic weapons deployed by each country to no more than 1,550 within seven years, was signed by Obama and Russian President Dmitry Medvedev in April.

Political analysts say Senate rejection of the treaty would be a major setback to warming ties between the Russia and the United States, giving ammunition to Russian hawks who oppose the thaw in relations with Washington.