US Under Secretary of State for Arms Control John Bolton met top Russian security and atomic energy officials and left the talks voicing optimism but revealing few results.
Washington again appeared unable to win Moscow's approval of a US-sponsored accord that would allow for the seizure of missiles and other potential components of weapons of mass destruction while they are being transferred at sea or in the air.
"You should ask the Russian government about that," Bolton told a reporter with a wry grin when asked what stands in the way of Russia joining the Proliferation Security Initiative -- also known as PSI -- proposed by US President George W. Bush last year.
Moscow has argued that PSI would open the way for unilateral military action from Washington.
It wants such deals to be negotiated through the UN Security Council in which it has veto power. Russia remains the only member of the Group of Eight leading industrialized nations that it recently joined not to have signed up to the PSI.
Bolton said that he was hoping that Moscow may change its mind during or shortly after a May 31-June 1 meeting of the PSI's 80 signatories scheduled to be held in Warsaw.
The hawkish Bolton regularly visits Russia -- though he is not always well-received here -- and has become one of Washington's top pointmen on issues dealing with Moscow's potential military trade and the spread of weapons of mass destruction.
Bolton was one of the key figures who helped negotiate a May 2002 arms reduction treaty signed by Presidents George W. Bush and Vladimir Putin in Moscow that was meant to reduce the two sides' nuclear arsenals by two thirds over 10 years.
But that treaty -- to Russia's immense displeasure -- now appears to have been dropped as Washington used a legal loophole to ignore the deal.
Little apparent progress was also made Thursday on Iran.
Bolton refused to directly answer a question during a press briefing on whether Russia intended to halt the construction of the first nuclear reactor in Iran.
He said that the United States remains "worried" that Russia may be helping Iran develop a ballistic missile program and that the Bushehr nuclear project might be used by Tehran "for its potential nuclear program."
"We certainly did discuss the subject of Iran in a number of different aspects.
"There may be tactical difference on the Iranian issues ... but there is no fundamental difference over the fact that Iran should not have a nuclear program.
"Iran is still pursuing a strategic decision to acquire nuclear weapons," he said, adding that Tehran has "never been fully cooperative" with UN weapons inspectors
But the head of Russia's atomic energy agency quickly countered Thursday that Moscow was sticking to all international rules while cooperating with Iran.
"We have said, and continue to say, that we are not breaking any rules by cooperating with Iran," Alexander Rumyantsev said.
Iran has remained a sore point in Russia-US relations despite a new wave of cooperation following the September 11, 2001 attacks on the United States.
Moscow has since appeared to have put the breaks on the project and delivered strong pressure on Iran to submit to open United Nations inspections of its potential military sites.
Iran's first nuclear reactor is now not due to become operational until 2005 -- years after schedule -- in a deal worth nearly one billion dollars (840 million euros) to Moscow. Russian authorities appear to have used several strategies to push it back to allay US concerns.