"Without doubt, NATO's expansion touches Russia's political, military and, to a certain extent, economic interests," Russia's top foreign ministry spokesman Alexander Yakovenko said in an official statement.

The terse statement was issued hours before Bush was due to welcome in the White House the prime ministers of Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia.

In a bid to relieve the tensions, NATO Secretary General Jaap de Hoop Scheffer was due to visit Moscow on April 7 to 8 for meetings with Russia's foreign and defense ministers, a source in the foreign ministry was quoted as saying by Interfax.

De Hoop Scheffer, in Washington for the accession ceremony, acknowledged there were still "some nuts to crack" -- referring to issues Moscow is sensitive about -- but stressed that NATO "needed a partnership with the Russians".

Moscow has been particularly incensed at the inclusion of the three Baltic states -- former Soviet republics that are still home to many ethnic Russians -- and the possibility that NATO troops will be stationed at its border.

Russia warned Monday that it may be forced to beef up its own defenses along the Baltic border in a move reminiscent of the Soviet Union's standoff with the US-led alliance.

"If we feel that this expansion poses a threat to us that demands a military response, this response will follow," Interfax quoted Deputy Foreign Minister Vladimir Chizhov as saying.

But besides the military threat, the expansion delivers another humiliating blow to Russia's international prestige, showing that former Warsaw Pact nations that once bowed to Moscow were now scrambling for Washington's support.

Russia is further concerned that NATO spy planes will soon be circling over the Baltics and peering into Russia's territory.

The Moscow statement Monday also underlined that the three states and Slovenia have not signed up to the Conventional Forces in Europeagreement that sets limits on the number of troops stationed in Eastern Europe.

The four countries did not exist as independent nations at the time of the treaty's signing and the limbo status could leave open the possibility of NATO stationing unlimited number of troops at Russia's western front.

"It is necessary to make sure that all of the alliance's new members will undertake to join the... CFE and until they do will undertake to follow its guidelines," Foreign Minister Sergei Lavrov was quoted as saying by Interfax.

Lavrov plans to attend a meeting of the NATO-Russia Council in Brussels Friday, the same day NATO will hold a flag-raising ceremony there for its new members.

Moscow conceded Monday that NATO has taken recent efforts at reforms, yet still viewed Russia as a military threat rather than an ally in international missions like the war on terrorism.

"We do not deny that recently, serious transformation has been happening in NATO. The number of troops and armaments is being reduced, and it is relying less on its nuclear arsenal," Yakovenko said.

"At the same time, our analysis shows that this transformation is happening slowly, at times haphazardly," said the Russian statement.

It added that the inclusion of the Baltic states meant that NATO "still believes that a war could break out in Europe."

Russia urged the Atlantic Alliance to "change its very nature" and begin to focus on how it could cooperate with Moscow.

The seven new states, represented by their prime minister, hand over their accession documents in Washington on Monday. A second ceremony on Friday will attended by their foreign ministers at the NATO headquarters. No Russian officials are expected to attend.

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