"Could they do more? The answer is yes," the official said, noting that Moscow has thus far refused to take part in a US-led scheme to seize such weapons in transit.

The official, speaking to reporters on condition of anonymity at the State Department, said Russia was "playing a little hard to get" in response to US calls for it to join the so-called "Proliferation Security Initiative" (PSI).

"They are so far not ready to join the process," the official said.

"They are interested but are raising lots of questions about what are the legal authorities that would permit this broader strategy of interdiction to go forward," the official said.

"We're trying to get them from this interrogatory mode into more active participation," the official said.

PSI was proposed by US President George W. Bush in late May with the aim of stopping countries -- particularly North Korea and Iran -- from buying or selling biological, chemical and nuclear weapons, their components or delivery systems.

Under the scheme, nations cooperate to intercept those items on the high seas, in international airspace or during overland transit.

The first PSI seizure -- of uranium enrichment components destined for Libya -- occured in October and may have helped to convince Tripoli to renounce weapons of mass destruction two months later, according to US officials.

Since its launch, the initial 11 members -- Australia, Britain, France, Germany, Italy, Japan, the Netherlands, Poland, Portugal, Spain and the United States -- have been joined by Canada, Denmark, Norway, Singapore and Turkey.

But Russia, along with China, has questioned the legality of the initiative, despite arguments from the United States and others that it is consistent with existing international law.

In addition, Russia has expressed displeasure with the fact that it was not invited to take part in the initial drafting of the initiative's outlines, according to the senior official.

"They are a little miffed that they weren't in on the ground floor, ... so they are playing a little hard to get," the official said.

Washington regards Moscow's participation in PSI as particularly important, given the size of its navy and its role in the Soviet era as a co-founder of the nuclear Non-Proliferation Treaty.

"Russia is a country with a big navy and could be an important player," the official said.

"Plus, Russia, as one of the founding members of the non-proliferation regime, would add political weight to the initiative and send a message to potential proliferators -- both sellers and buyers -- that they should stop messing around," the official added.

WAR.WIRE