US President Barack Obama's administration sought Thursday to allay Republican doubts about its new nuclear arms reduction treaty with Russia in a bid to ensure it is eventually ratified.

"There were no, and I repeat no, secret deals made in connection with the new START treaty, not on missile defense nor on any other issue," US arms control expert Rose Gottemoeller told senators during a hearing on the new Strategic Arms Reduction Treaty.

John McCain, a Republican senator who ran against Obama in the 2008 presidential campaign, said there were "puzzling and troubling references to missile defense" in the preamble and body of the treaty.

He added that such language appeared even though the administration initially promised there would be no references to the US missile defense project, a legacy of Republican president George W. Bush's administration.

Russia has said it reserves the right to withdraw from the treaty if Washington presses ahead with missile defense systems in Europe in a way that Moscow opposes.

In her opening remarks, Gottemoeller, the assistant secretary of state for verification, compliance, and implementation, also said the treaty contains "extensive verification" steps to prevent possible cheating by the Russians.

McCain, in his opening remarks, said the treaty raised serious questions about verification measures.

Republican senators could deny the administration the two-thirds majority it needs to win ratification for the successor START treaty that was signed by Obama and Russian President Dmitry Medvedev in April.

The administration hopes the new treaty, which replaces the Cold War era treaty that expired in December last year, will be ratified this year.

During testimony, McCain also asked Gottemoeller to comment on a State Department report that raised doubts about Russian compliance with the Chemical Weapons Convention.

She said she was convinced the Russians are "trying to resolve compliance concerns" in this area, but McCain expressed surprise that they should be trying to resolve issues that should have been settled years ago.