President Bush said Russia would not be able to veto new members
US President George W Bush has voiced his support for Ukraine's membership of Nato, during a visit to Kiev.
Sitting beside President Viktor Yushchenko, Mr Bush said Kiev had made a bold decision to request membership and the US "strongly supported it".
Mr Bush later arrived in Romania, where he has said he plans to press Nato allies to support Membership Action Plans for both Ukraine and Georgia.
Russia is fiercely opposed to the eastward expansion of Nato.
Deputy Foreign Minister Grigory Karasin told a hearing of the lower house of the Russian parliament, the Duma, that Ukrainian membership of the Western alliance would "entail a deep crisis in Russian-Ukrainian relations".
He said Ukraine would become a buffer between Europe and Russia.
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Afghanistan: 3 soldiers
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Mediterranean patrols in 2007
The West, he added, had to make a strategic choice because "this crisis will also affect in the most adverse way pan-European security too".
French Prime Minister Francois Fillon appeared to agree with the Russian assessment during a radio interview on Tuesday.
"We think that it is not a good answer to the balance of power within Europe and between Europe and Russia," he said.
A few thousand demonstrators shouted anti-Nato slogans in the centre of Kiev on Monday evening before Mr Bush's arrival.
A court ban on protests meant that number had dwindled to a few hundred as his motorcade drove through the streets on Tuesday morning.
Opinion polls in Ukraine suggest there is little public support for Nato membership.
The US president was welcomed by an honour guard before talks with President Yushchenko during which Mr Bush said they "spent a lot of time talking about Nato".
President Bush said that Russia would not have a veto over Kiev's role in Nato. He said Ukraine was currently the only non-Nato member supporting every Nato mission.
The Ukrainian leader was critical of Moscow's attempts to stop his country joining the alliance.
"I don't want the basic, fundamental principle... of open doors to the alliance to be changed for, excuse me, the veto power of a nation that is not a member of the alliance," Mr Yushchenko said.
After visiting Kiev, President Bush travelled to Nato's annual summit in Bucharest to press the case for eastern expansion. He will meet Russian President Vladimir Putin on Sunday.