The end of full-time, on-site access will likely ignite complaints in Congress, with insiders from both parties arguing over whether the George W. Bush or the Obama administration is responsible.

Republicans are worried by the previously undisclosed agreement between the Obama administration and the Kremlin in October, which formalizes the inspectors' departure this Saturday. This, they warn, would cripple Washington's ability to police Moscow's compliance with agreed reductions in its nuclear arsenal.

Democrats, on the other hand, insist they were "stuck" with an agreement reached late last year between the Bush administration and Moscow but not made public. This, they said, left the Obama team no choice.

The U.S.-staffed Votkinsk Portal Monitoring Facility operates under the terms of the Strategic Arms Reduction Treaty (START I) at the Votkinsk Machine Building Plant, about 600 miles east of Moscow -- the site where all Russian intercontinental ballistic missiles (ICBMs) are built.

The monitoring facility's present mandate ends with START's expiration Saturday, and the Obama administration has decided not to seek another agreement allowing Americans to remain, administration and congressional officials said.

"U.S. and Russian officials signed on Oct. 20 a series of documents, which establish the procedures to be followed for the completion of U.S. monitoring activities at the Russian ICBM production facility at Votkinsk," a State Department official said.

The two countries first agreed to "continuous monitoring" under the 1987 Intermediate-Range Nuclear Forces Treaty, signed by President Reagan and Soviet leader Mikhail Gorbachev.

The reciprocal site to Votkinsk in the United States was the former Hercules Aerospace missile production facility in Magna, Utah, which the Russians left more than eight years ago.

Although the United States does not produce new long-range missiles, Russia continues to do so and has built dozens of missiles since the monitoring started15 years ago. START banned certain types of missiles, which Americans at Votkinsk verified by counting and inspecting every missile that left the facility, analysts said.

However, the head of Russia's strategic missile forces, Nikolai Solovtsov, was recently quoted by Russian news agencies as saying that the assembly and deployment of next-generation RS-24 missiles would start once the treaty expires. Analysts said that could happen, because Moscow was not banned from developing new missiles.

During a visit to Moscow by President Obama in July, both countries agreed to draft a new arms-control treaty that would replace START. They also set a goal of cutting the number of strategic nuclear warheads to between 1,500 and 1,675 within seven years.

It now appears unlikely that the two countries will meet a self-imposed Dec. 5 deadline, but State Department spokesman Ian Kelly said Monday the U.S. hopes to have a draft treaty by the end of the month.

"Two main priorities here are reductions in nuclear arsenals and also preserving the verifications and monitoring mechanisms that are at the heart of the START treaty," Mr. Kelly said.

Congressional officials said they were told by the Obama administration that it "got stuck" with a deal made by the Bush administration to close the monitoring facility at Votkinsk. They also said the Bush administration did not want to extend START at all.

Paula A. DeSutter, assistant secretary of state for verification, compliance and implementation in the Bush administration, said she sent the Russians a post-START proposal in November 2008, but it was not a negotiated agreement.

She confirmed that it did not include continuing the Votkinsk mission, but attributed that to the Bush team's decision "not to limit delivery vehicles," so it did not need to count every missile Russia produced. "We didn't need the entire verification regime from START," she said.

In contrast, the Obama team "accepted the START approach to limit both warheads and missiles," so it made sense for them to keep Votkinsk, she said.

"There was nothing in our proposal that precluded the Obama administration from adding Votkinsk or any other verification measure, had they decided to take that approach," Ms. DeSutter said.

However, Daryl Kimball, executive director of the Arms Control Association, said the Obama team could not have gone back to renegotiate last year's accord.

Obama administration officials said they take Moscow's missile program seriously, and they are negotiating other verification measures to replace the permanent U.S. presence at Votkinsk. They declined to be more specific.

Mr. Kimball said that other ways to find out how many missiles Russia produces include regular inspections, data exchanges and intelligence gathering, such as tracking missile test flights.

"How significant [the loss of Votkinsk] is depends on what other monitoring mechanisms will be worked out," he said.

Some Republican members of Congress, however, have warned of major negative consequences, saying that nothing can replace everyday on-site monitoring, and that the administration should have sought to extend it under a new agreement.

U.S. officials are trying to complete a "bridging" agreement in Geneva this week, so some verification measures can remain in effect until a START replacement is finalized. The Votkinsk monitoring team, however, will leave, officials said.

"When Votkinsk goes away, Russia could deploy hundreds of missiles," said one senior Republican Senate aide. "Russia is a big country with many satellites passing overhead," so it will not be easy to count missiles based on test flights. "We are worried about what Russia will do that we are not going to know."

Another Republican aide said the "whole point of arms control" was to allow the United States to learn more about Russia's force strength than it could by just estimating it.

"We were radically bad about estimating, but we became better at understanding our adversary" after START and other treaties, he said. "You can't count mobile missiles from space."

The Obama administration points out that the two countries are no longer enemies and that it has gone out of its way to improve relations with Russia. "The nature of our relationship has changed, and we have a pretty good idea about where Russia's missile program is headed," one U.S. official said.

Still, critics say ties are not at a level where Washington can fully trust Moscow.

"For the first time in 15 years, an extensive set of verification, notification, elimination and other confidence-building measures will expire" on Saturday, Sen. Jon Kyl, Arizona Republican, said on the Senate floor late last month, suggesting that START should have been extended.

Mr. Kimball said another reason the Bush administration should take at least some responsibility for losing access to Votkinsk is that it "did not object to Russia's development of the RS-24 and did not favor the continuation of other types of legally binding verification provisions based on those in START."

"Senator Kyl was not on the floor of Senate railing against the Bush administration's decision not to continue essential START monitoring and verification provisions back in 2008, but now he's complaining that the Obama administration is not doing enough to maintain effective monitoring of U.S. and Russian strategic nuclear weapons," Mr. Kimball said. "He was against it before he was for it."

A senior aide to Mr. Kyl said the senator always cared about verification, but he is more vocal now because Mr. Obama wants to cut the two countries' nuclear arsenals well below Mr. Bush's target level of 1,700 to 2,200 warheads.

"If you are going to predicate the prudence of the cuts on the Russians' [compliance], you have to make sure you have high degree of confidence" that they are abiding by the limits, he said.