Seven former Soviet republics and East bloc states formally joined Nato on Monday as their prime ministers deposited "instruments of accession" to the alliance's founding treaty here.
Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia became members of the North Atlantic Treaty Organisation in a ceremony at the US Treasury, cementing the largest expansion in the now 26-member group's 55-year history.
US Secretary of State Colin Powell presided over the ceremony marking the first time Nato has embraced former Soviet states - the Baltic nations of Estonia, Latvia and Lithuania - and firmly shifted its focus eastward, raising concerns in Russia.
"To the seven heads of states here assembled, I say to you and to your people: Welcome to the greatest and most successful alliance in history. Welcome," he said.
Powell said the expansion was a "historic step" in achieving a vision to extend "Europe's zone of freedom and security from the Baltics to the Black Sea."
"Nato is determined above all to prevent aggression," he said. "Now it is determined above all to promote freedom, to extend the reach of liberty and to deepen the peace.
"I am confident that with the new energy that these seven nations bring to our alliance, our alliance will be as successful in the future as it has been in the past."
The seven prime ministers - in addition to the premiers from three Nato aspirants: Albania, Croatia and Macedonia - are to join US President George W. Bush at a White House event later Monday to mark the occasion.
The United States is the depository nation for the Washington Treaty that created the North Atlantic alliance in 1949 at the height of the Cold War and the "instruments of accession" are to be held by the State Department.
Earlier Monday, Moscow bristled at the accession of the Baltics even as Nato Secretary General Jaap de Hoop Scheffer said friction with Russia would be minimised despite the imminent start of patrols there by alliance aircraft out of Lithuania.
"Without doubt, Nato's expansion touches Russia's political, military and, to a certain extent, economic interests," Russian foreign ministry spokesman Alexander Yakovenko said in a terse statement.
In addition to the fact that the Baltic states are still home to many ethnic Russians, the expansion deals a blow to Russia's international prestige as former Warsaw Pact nations that once bowed to Moscow now turn to the West.
But de Hoop Scheffer, who is to visit Moscow on April 7 to 8, said he did not believe the enlargement would create greater difficulties in relations with Russia although he acknowledged there were "some nuts to crack" in relations with the Russians.
"Nato needs a partnership with the Russians," he told defence reporters here. "It's in Nato's interest and at the same time it is in Russia's interest that we have a strong partnership."
He said Russia understood Nato had "no ulterior motives" in policing the Baltic airspace.
Despite Moscow's grumbling, the admission of seven new members has been met with little of the impassioned debate that accompanied the last round of Nato enlargement in 1999 when the Czech Republic, Hungary and Poland joined.
Softening Russia's antagonism have been assurances from Nato that is merely replacing Cold War defences aimed at Moscow with light, rapidly deployable forces designed to respond to crises outside Europe, notably in the Middle East and Central Asia and non-state terrorism.