The North Atlantic Treaty Organisation will formally increase to 26 members after Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia hand over accession documents to the US government.

Bush will later greet the prime ministers of the former East bloc and Baltic republics along with NATO Secretary General Jaap de Hoop Scheffer, who acknowledged there are "some nuts to crack" in relations with Russia but insisted there was no need for tensions.

"As NATO acts to face the new challenges of the 21st century, the membership of these seven nations in NATO will advance the cause of freedom, and strengthen the Atlantic Alliance, the central pillar of transatlantic relations," the White House said in a statement.

"These central and east European democracies have already acted as allies through their strong solidarity and actions in the war on terrorism, and in helping to strengthen peace and democracy in Afghanistan and Iraq," the White House said.

But 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.

The enlargement is the biggest in NATO's history 55-years ago at the height of the Cold War.

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.

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."

Albania, Croatia and Macedonia also want to join NATO, and Bush will also meet their prime ministers Monday.

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