US President Barack Obama's administration urged the Senate Thursday to back a new nuclear weapons treaty with Russia, telling Republicans the pact would not undermine US missile defense plans.

"From the very beginning of the negotiations, this administration has been very clear. This treaty limits strategic offensive nuclear arms, not missile defenses," Secretary of State Hillary Clinton told a Senate hearing.

"We share a strong belief that the new start treaty will make our country more secure and we urge the Senate to ratify it expeditiously."

Clinton and other key members of Obama's administration argued before the Senate Armed Services Committee for ratification of the Strategic Arms Reduction Treaty (START), signed with Russia on April 8.

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.

Republican Senator John McCain said the Russian statements are "bound to be worrisome to anyone" and sounded unconvinced by explanations offered by Clinton, Defense Secretary Robert Gates and Admiral Mike Mullen, chairman of the US Joint Chiefs of Staff.

"It's clear from many statements that the Russian leadership has made that there is a very different interpretation of this treaty, from what has been stated here, concerning the connection to missile defense systems and that of the Russians," he said.

But Clinton said the statement made by Russia "does not limit or constrain our missile defense efforts."

"Indeed, a US unilateral statement makes it clear that, quote, 'our missile defense systems are not intended to affect the strategic balance with Russia,'" she said.

The administration has stated that the United States plans to improve and deploy missile defense systems to defend against a limited attack, and not against Russia's vast arsenal, Clinton and Gates told the panel.

Moscow has always opposed US missile defense projects, dating back to the 1970s, said Gates, a former CIA analyst and agency director.

"The Russians can say what they want, if it's not in the treaty, it's not binding on the United States," he said.

But Gates said even Russia's unilateral statement "hedged," giving Moscow the option to accept US missile defense weaponry if it is not aimed at Russia's nuclear force.

Under the new pact, each nation will be allowed a maximum of 1,550 deployed warheads, about 30 percent lower than a limit set in 2002.

They are also restricted to 700 air, ground and submarine-launched nuclear intercontinental ballistic missiles.

But Republicans have cautioned they will oppose the pact if they think it will hamper US missile defense.

Treaty ratification needs 67 votes, but Democrats and their two independent allies hold only 59 seats in the 100-member Senate, meaning they will need to rally at least eight Republicans to their side.