WASHINGTON - Spies, senators and an alleged arms dealer dubbed the "Merchant of Death" all appear to be working against one of US President Barack Obama's few foreign policy success stories: the "reset" in relations with Moscow.

Russia's Foreign Ministry protested sharply after Viktor Bout was flown from Thailand to the United States on Tuesday, ending a two-year battle over the 43-year-old former Soviet air force officer.

The State Department, which pushed for Bout's extradition, said it was confident it would have no effect on the two-year old US drive to "reset" strained ties with Moscow.

"We have a broad and deep relationship with Russia," State Department spokesman P.J. Crowley told a news briefing.

"Sometimes we have tensions that crop up periodically and we work to manage those. I don't expect that this will have any impact on our relationship with Russia."

But Bout's arrival comes at a sensitive moment in US-Russia ties as Obama prepares to meet Russian President Dmitry Medvedev at a NATO summit amid doubts over the US president's ability to deliver Senate confirmation of the new START nuclear arms deal between the two countries.

Those doubts deepened on Tuesday when a key Republican senator voiced new reservations about the treaty, spurring Vice-President Joe Biden to warn that failure to pass it would threaten US national security.

"The new START treaty is a fundamental part of our relationship with Russia," Biden said in a statement.

Political analysts said the outlook was worrying for US-Russia relations.

"The reset policy has been hailed as the administration's biggest success, but this steady drip of negative news may begin to affect that perception," said Heather Conley, a Russia expert at the Center for Strategic and International Studies, a Washington think-tank.

"There is definitely a feeling that clouds are beginning to gather."

The Russia reset, which Obama launched after taking office in January 2008, has yielded dividends for both Washington and Moscow as Russia joined the United States to put pressure on Iran over its nuclear program and became a helpful partner for the US-led war against Taliban insurgents in Afghanistan.

Russia, for its part, has won increased US backing for its economic reforms, including its bid to join the World Trade Organization.

COLD WAR TENSIONS LINGER

But reminders of past tensions between the two former Cold War foes are never far from the surface.

News this month that the head of Moscow's deep-cover spying operations defected after betraying the network in June was a humiliating setback that spurred cloak-and-dagger headlines around the world.

The extradition of Bout, who was arrested in Bangkok in March 2008 in a US-led sting operation, was another unhappy surprise for Moscow, which had warned repeatedly that his case was politically motivated and could damage US-Russia ties.

An inspiration for the Hollywood movie "Lord of War" starring Nicholas Cage, Bout faces US accusations of trafficking arms since the 1990s to dictators and conflict zones in Africa, South America and the Middle East.

Samuel Charap, a Russia expert at the Center for American Progress, a liberal think tank, said Washington's determination to bring Bout to trial may sow fresh doubts about the United States' commitment to the broader reset policy.

A bigger worry, however, is continued Senate delay on START, which Obama and Medevedev signed in April, committing to cut deployed nuclear warheads by about 30 percent.

"START is the cornerstone, and it is a demonstration to the Russians on whether Obama can deliver, and whether the US is really interested in pursuing shared interests cooperatively," Charap said.

While both Obama and Secretary of State Hillary Clinton have urged the Senate to ratify the measure during its "lame duck" session in coming weeks, those hopes were hit hard on Tuesday when a key Republican said that did not leave enough time to debate.

The comments by Senator John Kyl, the number two Republican in the Senate, could push a START vote to next year, when passage may be more difficult because Democrats emerged with a smaller Senate majority following recent elections.

"It is going to be a real test of the president's political capital in Washington to get (START) done," said Conley of CSIS. "The question will be: does the president have the ability to move his foreign policy agenda forward."