NATO Secretary General Jaap de Hoop Scheffer said he 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."
De Hoop Scheffer said it was good sign that Russian Foreign Minister Sergei Lavrov plans to attend a meeting of the NATO-Russia Council in Brussels Friday, the same day NATO will formally welcome in its new members.
The NATO chief said he planned to visit Moscow in early April, and would see Russian Defense Minister Sergey Ivanov Monday at a Russia-NATO meeting on terrorism in Norfolk, Virginia.
De Hoop Scheffer spoke shortly before the seven new members deposited instruments of accession to NATO at a ceremony here, effectively expanding the alliance to 26 members.
The new members include four from the former East bloc -- Romania, Bulgaria, Slovakia and Slovenia -- and the Baltic states of Estonia, Lithuania and Latvia.
Admission of the Baltic countries has been the bitterest pill for Moscow to swallow.
Russian officials last week warned that Moscow might build up its nuclear forces in response to the expansion, and expressed concern over NATO air patrols over the Baltics, which de Hoop Scheffer said were set to begin Monday.
"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 reaffirmed Monday in an official statement released in Moscow.
The statement underlined that the three states and Slovenia have not signed up to the CFE as they did not exist as independent nations when the treaty was signed. The limbo status could leave open the possibility of NATO stationing unlimited number of troops at Russia's western front.
Moscow also fears NATO air patrols over the Baltics will be used to spy on its territory.
De Hoop Scheffer said the decision to use NATO fighters to patrol the Baltics was fully explained to Lavrov when it was taken two weeks ago by the alliance's decision-make 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.