Nato Secretary-General Jaap de Hoop Scheffer said the alliance would tell Russia in a meeting today the Ukrainian people must have the chance to elect their leader in fair elections.
There are growing tensions over Ukraine between the west and Russia, which has warned western nations against interfering there and in other ex-Soviet republics.
Mr de Hoop Scheffer was asked what he would tell Russian Foreign Minister Sergei Lavrov during talks on military and security cooperation at Nato's headquarters. He said: "On Ukraine, the message will be what it has been since the beginning: that the Ukraine people decide in free and fair elections who their president should be.
"I think yesterday's developments are a very good step in that direction," he added, referring to laws passed by the Ukrainian parliament on Wednesday aimed at ensuring a fair rerun on December 26th.
Mr Lavrov will meet US Secretary of State Colin Powell at the meeting of the Nato-Russia Council. The two clashed over Ukraine at a conference in Sofia, Bulgaria on Tuesday, with Mr Powell dismissing Russian fears of US meddling in the region.
Nato and Russia are due to make two announcements at their meeting on Thursday - a joint action plan on fighting terrorism together, and the formal launch of Russian support for Nato's shipping surveillance mission in the eastern Mediterranean.
Luxembourg's prime minister, who takes over the presidency of the 25 nation European Union next month, was quoted today as saying he feared referendums on the bloc's planned constitution would produce policy stalemate.
In an interview with Germany's Die Weltnewspaper, Luxembourg Prime Minister Jean-Claude Juncker said he no longer opposed offering EU membership talks to Turkey but warned against doing the same for Ukraine.
"I can only warn against offering Ukraine the prospect of full membership," Juncker said. "We need a special relationship with Ukraine that does justice to its strategic importance."