The new START arms control treaty imposes no limits on US missile defense weapons despite concerns voiced by Russia, President Barack Obama's deputies told lawmakers on Tuesday.

At a senate hearing on the START treaty, Secretary of State Hillary Clinton and Defense Secretary Robert Gates sought to counter criticism from some Republicans that the agreement could undermine US plans for missile defense.

"Nothing in the treaty will constrain our missile defense efforts," Clinton said.

The United States was "not bound" by a Russian statement opposing missile defense and had declared plans "to continue improving and deploying its missile defense systems," she said.

Obama and his Russian counterpart Dmitry Medvedev signed the new arms reduction deal in an elaborate ceremony in Prague in April.

But the treaty must be ratified by the Senate, where it faces Republican opposition.

Republican Senator Bob Corker expressed skepticism, saying Moscow had a different interpretation of how the treaty applied to missile defense.

"Shouldn't it trouble us that, before we ever get started, that each of the countries has a very different opinion of what we've negotiated as it relates to missile defense?" Corker said.

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

But Gates dismissed concerns about Russian opposition, saying Moscow has always objected to anti-missile programs.

The defense secretary, a Washington veteran and former CIA director, said "the Russians have hated missile defense ever since the strategic arms talks began in 1969."

"And so the notion that this treaty has somehow focused this antagonism on the part of the Russians, toward missile defense, all I would say is it's the latest chapter in a long line of Russian objections to our proceeding with missile defense," he said.

He said Russia had always opposed it probably "because we can afford it and they can't."

Gates cited major funding in next year's budget to bolster US missile defense systems as proof that "we are putting our money where our beliefs are."

The budget for 2011 sets aside 19.9 billion dollars for missile defense radars, interceptors and launch sites.

The testimony came as a new study questioned the Pentagon's portrayal of the anti-missile program, saying it was based on "technical myths."

Two university scientists reviewed 10 tests of the SM-3 "kill vehicle," designed to take out ballistic missiles, and concluded that the interceptor succeeded in directly hitting mock warheads in only one or two cases, according to the latest issue of "Arms Control Today."

But the US Missile Defense Agency on Tuesday rejected the study findings, calling them "flawed, inaccurate and misleading."

Gates and Clinton were joined at the hearing by the chairman of the Joint Chiefs of Staff, Admiral Mike Mullen, who also told senators that the treaty would have no impact on missile defense programs.

Clinton acknowledged treaty language that bars the conversion or use of offensive missile launchers for anti-missile interceptors.

But she said the US military never planned to use those existing launchers and instead wanted to build smaller, cheaper missile defense silos.

The START treaty would significantly cut US and Russian nuclear weapons to about 1,550 each.

That figure represents a 74 percent reduction from the arsenal size agreed in the original START deal, signed in 1991, which expired at the end of 2009.