WASHINGTON, March 29: Seven eastern European allies joined Nato in a triumphant ceremony on Monday, but the expansion could slow deployments and has angered Russia by shifting the 55-year-old trans-Atlantic alliance to its borders.

The entry of Estonia, Latvia, Lithuania, Bulgaria, Romania, Slovakia and Slovenia increased the members in the U.S.-dominated alliance to 26. Nato's chief immediately sought to ease Russian fears over the historic expansion of the alliance.

And in a reflection of the shift eastward of an alliance forged to fight the Cold War, Nato fighter jets headed to the Baltics, Secretary-General Jaap de Hoop Scheffer said.

"Welcome to the greatest and most successful alliance in history," U.S. Secretary of State Colin Powell told the nations' prime ministers, who formally handed over their accession documents.

Despite fears the enlargement could hamper timely deployments because Nato needs consensus on military action, the top U.S. diplomat also said he supported the ambitions of Albania, Croatia and Macedonia to one day join the alliance.

The new members exulted in joining an organization which ensures military protection to the 26 nations. "Today, it is really fantastic day for Slovakia. ... I consider this a very big success," Foreign Minister Eduard Kukan told Reuters.

Forty percent of Nato will now be former communist states. Russia has bitterly criticized the enlargement, especially into nations that formed part of the Soviet Union until 1991.

On accession day, a Russian parliamentary deputy dismissed the Washington ceremony to formally receive the seven allies' acceptance documents as a "show." frosty welcome: Russia has given a frosty welcome to the alliance expansion on its doorstep.

"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 released in Moscow.

Russia is particularly concerned about the inclusion of the Baltic states, Estonia, Latvia and Lithuania, all former Soviet republics which are still home to many ethnic Russians, and the possibility that Nato troops will be stationed at its border.

The expansion is also a blow to Russia's international prestige as former Warsaw Pact nations that once bowed to Moscow now turn to the West. The Nato secretary general, who said he would go to Moscow in April, did not believe the expansion would cause new tension with Russia but acknowledged there were problems over the Conventional Forces in Europe (CFE) treat which limits troop numbers in eastern Europe.

"There are some nuts to crack, of course," he said. "When I say we have some nuts to crack it's, of course, Russian worries about the effectiveness of the CFE treaty. Nato worries about the Russians still having their forces in Moldova-Transdniestra and Georgia," he said.

Nevertheless, he said, "Nato needs a partnership with the Russians. It's in Nato's interest and at the same time it is in Russia's interest that we have a strong partnership."

Nato air defence patrols over the Baltic republics - another source of controversy with Russia - were to be ready from Monday. De Hoop Scheffer said the decision to use Nato fighters to patrol the Baltics was fully explained to Russia's Foreign Minister Sergey Lavrov when it was taken two weeks ago by the alliance's decision-making North Atlantic Council.

"At this very moment fighters are in the air to land at Lithuania airport very shortly," he said. "It's Nato airspace and Nato airspace has always been patrolled and covered, which will always be the case when later today the alliance will be formally enlarged by seven new member states," he said.

"UNFRIENDLY" EXPANSION: President George Bush, criticized for paying scant attention to alliance-building, will also host the countries' prime ministers for a White House ceremony.