The prime ministers of Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia deposited "instruments of accession" to the North Atlantic Treaty Organization's founding Washington treaty and brought the number of members to 26.

"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," said Colin Powell, the US secretary of state.

He 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," Powell said. "Now it is determined above all to promote freedom, to extend the reach of liberty and to deepen the peace."

President George W. Bush met the leaders of the new members at the White House later, along with the prime ministers of Albania, Croatia and Macedonia, which also want to join NATO.

Bush seized the opportunity to highlight the US war on terrorism.

"Today, our alliance faces a new enemy, which has brought death to innocent people from New York to Madrid. Terrorists hate everything this alliance stands for. They despise our freedom. They fear our unity," he said.

"They seek to divide us. They will fail."

Russia again expressed disquiet over the biggest increase in NATO membership since it was founded at the height of the Cold War in 1949.

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

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

Russia is particularly concerned about the inclusion of the Baltic states, Estonia, Latvia and Lithuania. All are former Soviet republics that could now provide a base for NATO troops on the Russian border.

"The Cold War is indeed over," said Romanian Prime Minister Adrian Nastase at a press conference of the seven leaders.

"Today is a great day for all our nations," said Latvian Prime Minister Indulis Emsis. "I express my gratitude to the United States government for the support our nation has experienced during all those long years of the Soviet occupation."

NATO Secretary General Jaap de Hoop Scheffer said air defense patrols over the Baltic Republics could start straight away. Russia fears these patrols could be used to spy on its territory.

De Hoop Scheffer, who will go to Moscow in early April, acknowledged there could be problems with Russia over the Conventional Forces in Europetreaty, which limits troop numbers in Eastern Europe. But he insisted there was no need for tensions.

The Baltic republics and Slovakia were not independent states when CFE was signed and Russian Foreign Minister Sergey Lavrov said all four will have to join the CFE and keep to its guidelines until they do.

"There are some nuts to crack, of course," said the NATO chief.

"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."

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."

De Hoop Scheffer said the decision to use NATO fighters to patrol the Baltics was fully explained to Russia's Foreign Minister Sergei Lavrov when it was taken two weeks ago by the alliance's decision-making North Atlantic Council.

"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.

The United States is the depository nation for the Washington Treaty that created the transatlantic alliance. The "instruments of accession" are to be held by the State Department.

A second ceremony will be held at NATO's Brussels headquarters on Friday.

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