Russia has stated its concern following a ceremony in which seven eastern European countries joined NATO in the military alliance's largest single expansion to date.
The entry of Estonia, Latvia, Lithuania, Bulgaria, Romania, Slovakia and Slovenia has increased the members in the US-dominated alliance to 26 and extends its reach to the Russian border.
Russia has bitterly criticised the enlargement, especially into nations that formed part of the Soviet Union until 1991. A Russian parliamentary deputy dismissed the Washington ceremony to formally receive the seven allies' acceptance documents as a "show."
President George W. Bush, criticised for paying scant attention to alliance-building, will also host the countries' prime ministers for a White House ceremony.
"It's understandable that the Americans are putting on a show today," Konstantin Kosachev, representative of a Russian parliamentary committee on international affairs, told journalists.
He said a NATO plan to patrol the airspace of the three Baltic states was an "unfriendly" move. Estonia and Latvia border Russia, while Lithuania has a frontier with Moscow's Kaliningrad enclave.
"It can not be ruled out that Russia ought to look at the possibility of taking corresponding measures."
Russian Foreign Ministry spokesman Alexander Yakovenko said: "The main thing that could improve the state of European security is a fundamental change in the very nature of NATO, a consistent implementation of the agreement of the new quality of the relationship between Russia and NATO."