Undersecretary of State John Bolton spent two days in Moscow meeting top defense and foreign ministry officials, but failed to convince them that Russia should join the Proliferation Security Initiative (PSI).

The agreement has been signed by all members of the Group of Eight leading industrial nations except for Russia. Washington would like to see Moscow on board by the time the United States hosts the next G8 summit in June.

But Bolton admitted that Moscow at the moment had too many questions about the PSI despite its participation in the US-led "war on terror".

Russia fears that the PSI initiative will allow the United States to launch unilateral raids without agreement from international institutions like the UN Security Council, where Russia has veto power.

The issue of Russia's role in international affairs has become particularly sensitive after the United States launched the war on Iraq despite Russia's opposition to the campaign.

Meanwhile Bolton is seen as one of the more hawkish officials within the US administration and appeared to receive a curt reception in Moscow during his visit.

He has previously played tough with Russia on issues concerning nuclear disarmament and Washington's recent decision to annul a Cold War-era agreement on preventing the construction of nation-wide nuclear defense shields -- an agreement based on the premise that a nuclear war would assure destruction of both countries and which therefore would never be launched.

Bolton ended his visit to Moscow by saying that "we are prepared to hold more discussions."

"I believe that they are certainly supportive of the PSI," he said of his Russian counterparts.

"And I think they are considering what their objectives will be," Bolton told reporters after meetings that included one with Atomic Energy Minister Alexander Rumyantsev.

But he admitted that Russia at the moment was concerned about how seriously it was being treated by the international community and whether Moscow would be seen as an equal partner in the anti-terror campaign of the United States and other Western nations.

"I don't think that the Russia position is that they are opposed to this (PSI).... I think they are studying the extent of the involvement they would like to have.

Bolton admitted that it was still not clear "whether they (the Russians) are full participants or not" in the war against terror. "We think that Russia brings a lot of advantages to the table."

But the message was far more skeptical when delivered by Russian officials.

"The explanation presented by the US side was far from satisfactory from the Russian point of view," Russian news agencies quoted a defense ministry official who attended Bolton's talks with foreign ministry officials as saying.

The source said that Russia "was unable to understand how the forceful interception of planes and ships that may be carrying weapons of mass destruction agrees with international law."

"We have also failed to receive the technical aspects" of how these operations would be conducted, the source said.

The source added that Russia would only sign up to the agreement "if this initiative answers our national interests."

Bolton's visit came just days after the stay of US Secretary of State Colin Powell, who published an article in a top Russian daily criticizing President Vladimir Putin's approach to the media and political freedoms in Russia.

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