NATO rejected on Friday Russian demands to withdraw its forces from Romania and Bulgaria as diplomatic efforts continued in an effort to prevent a war in Europe after Russia amassed tens of thousands of troops near Ukraine's borders.

"NATO will not renounce our ability to protect and defend each other, including with the presence of troops in the eastern part of the alliance," NATO spokesperson Oana Lungescu said in a statement.

Moscow has demanded legally binding guarantees from NATO that the bloc will stop its expansion and return to its 1997 borders.

Earlier on Friday, the Russian foreign ministry said the security guarantees that Moscow seeks from the West include provisions requiring NATO forces to leave Romania and Bulgaria.

Replying to a question about what that would mean for Bulgaria and Romania, which joined NATO after 1997, the ministry said Russia wanted all foreign troops, weapons and other military hardware withdrawn from those countries.

NATO rebuffed these demands, saying they would create first and second class NATO members, which the alliance could not accept.

"We reject any idea of spheres of influence in Europe. We will always respond to any deterioration of our security environment, including through strengthening our collective defense," the NATO spokesperson said.

"NATO is vigilant and continues to assess the need to reinforce the eastern part of our Alliance."