"We didn't want this enlargement, and we will continue to maintain a negative attitude," Foreign Minister Sergei Lavrov said after talks with NATO counterparts including US Secretary of State Colin Powell.
"It's a mistake," he told reporters, on the day that NATO held a welcome ceremony for its new members -- Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia.
"The presence of American soldiers on our border has created a kind of paranoia in Russia."
For his part, Russian President Vladimir Putin toned down Lavrov's ire by saying relations were "developing positively" between Moscow and NATO.
"But the encroachment of NATO military infrastructure to our borders is being carefully studied by our specialists," Putin said after talks with German Chancellor Gerhard Schroeder.
"And we will build our military position from these conclusions."
Moscow has shown particular anger at the inclusion in NATO of the Baltic states Estonia, Latvia and Lithuania, which are all former Soviet republics.
Russia warned Monday that it may be forced to beef up its own defences along the Baltic border in a move reminiscent of the Soviet Union's standoff with the US-led alliance during the Cold War.
With NATO F-16s starting patrols of the Baltic trio's airspace this week, Russia went as far as to warn of a possible "military response".
The North Atlantic Treaty Organisation and Russia are also embroiled in a row over the Conventional Forces in Europe (CFE) treaty, which limits troop numbers in eastern Europe.
The Baltic republics and Slovenia were not independent states when the CFE was signed in 1989 and Russia says the new NATO members will now have to join the treaty, and keep to its guidelines until they do.
"We have received today assurances, but we haven't received guarantees, that they will sign up to the CFE treaty," Lavrov said after meeting the NATO ministers.
NATO Secretary-General Jaap de Hoop Scheffer, who is due to visit Moscow on April 7-8 in a bid to clear the air, played down the sense of crisis.
"I was heartened by the discussions today... which made clear that we can all benefit from this partnership," he told reporters.
"The fact that Minister Lavrov came to Brussels was a good sign."
Powell, in an interview published Friday with newspapers from the seven new NATO members, called on Russia in turn to do more to comply with its own military obligations in Europe.
He said he had spoken with Lavrov on Wednesday to reassure him that deployment of the F-16 fighters was just "to essentially bring the new nations of NATO under NATO air cover".
"I don't sense that the Russians will find it necessary to counter this move with anything that would be either provocative or destabilizing or dangerous," he said, according to a State Department transcript of the interview.
Powell countered Lavrov's objections over the CFE by pointing to the continuing deployment of Russian troops in Georgia and Moldova, despite promises to pull out.
"They are of course very interested in seeing everyone ratify the adapted CFE treaty, but we made it clear to the Russians that we believe a pre-condition for that is for them to comply... and withdraw from Moldova and Georgia," he said.
"They're clearly linked, and this is a precondition before we can expect the other countries to ratify the adapted CFE treaty and that will remain our position."