A top Russian MP on Wednesday expressed doubt that the US Senate would ratify the START nuclear arms reduction treaty, the first time a top Moscow official has sounded alarm on the accord failing.

President Barack Obama and Russian counterpart Dmitry Medvedev signed the treaty earlier this year but it still requires ratification by the US and Russian legislatures.

Obama has urged the Senate to approve the treaty in its so-called "lame-duck" session after elections, before a new Senate takes its place with stronger Republican representation.

"The problem is that the process of ratification in the United States overlapped with the elections," Konstantin Kosachyov, who heads the Russian lower house's foreign affairs committee, said according to Interfax.

"The agreement is not going to be agreed in the lame duck session," he added. "Overall, my forecast is that there is a 50-50 chance of ratification."

Until now, the Russian foreign ministry has been careful not to give any impression that the new START treaty risks failing.

But Kosachyov's statements are often seen as reflecting the foreign ministry's viewpoint.

He said that if the treaty is not ratified there will be a "strange" situation as no mechanism will exist for carrying out inspections of the other side's missile arsenals.

The Strategic Arms Reduction Treaty -- signed by Medvedev and Obama in April -- restricts each nation to a maximum of 1,550 deployed warheads, a cut of about 30 percent from a limit set in 2002.

The Russian lower house of parliament, the State Duma, has indicated it will rubber stamp the treaty only after its ratification in the United States.