WASHINGTON — U.S. knowledge of Russia's nuclear capabilities will dwindle if the New START nuclear arms treaty with Moscow is not ratified, the chief U.S. negotiator argued as a Senate panel on Friday scheduled a vote on the document.

New START is one of the central planks of U.S. President Barack Obama's nuclear policy and part of his effort to "reset" relations with Moscow. Obama wants it ratified by the Senate this year. But some Republican support will be needed, and so far, little has emerged.

Assistant Secretary of State Rose Gottemoeller contended that the longer it takes to ratify the new agreement, the less Washington will understand about Russia's nuclear arsenal.

Inspections of bases inside Russia stopped when the old START treaty expired in December.

"U.S. knowledge of Russian nuclear forces will substantially erode over time if the treaty is not ratified and brought into force, increasing the risk of misunderstandings, mistrust, and worst-case analysis and policymaking," Gottemoeller wrote in the forthcoming issue of Arms Control Today.

She was the chief U.S. negotiator on the treaty, which would cut the number of nuclear warheads deployed by the United States and Russia by about 30 percent.

The Senate Foreign Relations Committee on Friday put the treaty on the agenda for its Sept. 16 meeting. New START is expected to pass the committee, which has a majority of Obama's Democrats as well as the one declared Republican supporter of the document so far, Senator Richard Lugar.

But the treaty has to have 67 votes to clear the Senate, meaning that it needs at least eight Republican supporters in addition to the Democrats.

President Dmitry Medvedev has told the State Duma to ratify the treaty just as soon as the Senate does.

In Washington, some Republicans are worried that it may limit U.S. missile defenses, while others want Obama to promise to spend more money modernizing the nuclear weapons that remain.

Republican criticism has increased as partisan rhetoric heats up before congressional elections on Nov. 2.

Gottemoeller pointed out that when the first START treaty expired last December, the United States became unable, for the first time in more than 20 years, to conduct nuclear arms inspections inside Russia.

Those inspections dated back to 1988 under the Intermediate-Range Nuclear Forces Treaty with the Soviet Union, and had become a vital means of verifying compliance, she said in the article for the September issue of Arms Control Today, the monthly publication of the Arms Control Association in Washington.

New START provides for up to 18 on-site inspections annually, while the old START provided for 28 annual inspections, Gottemoeller acknowledged.

However, there are only half as many Russian nuclear weapons facilities to inspect as before, because some had been shut down, or were located in Belarus, Kazakhstan and Ukraine, former Soviet republics that no longer deploy strategic offensive nuclear arms, Gottemoeller wrote.