The prime ministers of Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia deposited "instruments of accession" to the North Atlantic Treaty Organisation'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, who called the move "historic".

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 was to meet the leaders of the new NATO members at the White House later in the day, along with the prime ministers of Albania, Croatia and Macedonia, which also want to join NATO.

But 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 top 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 former Soviet republics which are still home to many ethnic Russians, and the possibility that NATO troops will be stationed at its border.

NATO Secretary General Jaap de Hoop Scheffer said that air defence patrols over the Baltic Republics could start straight away. Russia fears these patrols could be use 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 Sergey 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 supreme allied commander, US General Jim Jones, said Friday that NATO "is in the process of one of its most fundamental changes in its history.

"It will be a different organization. It will have a different membership. The Eastern European influence will change the voting demographics. It will bring different views," Jones said. NATO "is going global instead of regional."

The United States is the depository nation for the Washington Treaty that created the North Atlantic alliance in 1949 and the "instruments of accession" are to be held by the State Department.

A second ceremony will be held at NATO's Brussels headquarters on April 2 with its 19 current members and the foreign ministers of its seven new members.

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