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.