"At this very moment fighters are in the air to land at Lithuania airport very shortly," NATO Secretary General Jaap de Hoop Scheffer told defense reporters here.
When the Baltic states of Estonia, Latvia and Lithuania, Romania plus Bulgaria, Slovakia and Slovenia deposit the instruments of accession to the alliance at the State Department later in the day "NATO air space will be covered," he said.
NATO's expansion and its decision to station fighter aircraft in Lithuania and extend air cover of the Baltics has drawn expressions of concern from Russia mixed with threats to build up its nuclear forces.
But De Hoop Scheffer said he did not believe the enlargement would create greater difficulties in relations with Russia.
He said the decision to extend NATO's air defenses to the Baltics had been fully explained to Russian Foreign Minister Sergey Lavrov, who was expected to come to Brussels Friday when the alliance formally marks the addition of the seven new members.
"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," he said.
He said Russia understood NATO had "no ulterior motives" in policing the Baltic airspace.
"If they would make this into a big thing, which they don't, minister Lavrov would certainly not have come to Brussels on Friday. I welcome his decision," he said.
NATO will be conducting the air patrols with Belgian fighter aircraft supported on the ground by Norwegian crews, Lithuanian officials have said.
The Baltics states have been encouraged to focus their military contributions to the alliance in areas such as peacekeeping, rather than acquiring expensive new fighter jets, the NATO chief said.
Russia raised its worries about the bigger NATO again on Monday, hours before US President George W. Bush was to meet the leaders of the seven new nations.
"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.
Moscow conceded that NATO has made efforts to change, yet still viewed Russia as a military threat rather than an ally in international missions like the war on terror.
"We do not deny that recently, serious transformation have been happening in NATO. The number of troops and armaments is being reduced, and it is relying less on its nuclear arsenal," Yakovenko said.
"At the same time, our analysis shows that this transformation is happening slowly, at times haphazardly," said the Russian statement.