Bush vowed full support for the bids on Tuesday despite vehement Russian opposition and French and German objections to allowing the former Soviet states to begin the NATO admission process.

Bush’s strong stance sets up a showdown in NATO, whose leaders will decide this week whether to give Ukraine and Georgia so-called “membership action plans”.

It may further complicate US and Russia’s already strained ties, which has been made worse by Moscow’s intense resistance to Washington’s plans to set up ballistic missile shields in Europe.

But Bush said Russia would not have a veto on what other countries do. He also rejected any trade-off on missile defence and NATO membership, and pledged to work “as hard as I can” to open NATO’s doors to Ukraine and Georgia.

“Both are ready and worthy to be welcomed,” he said. “Your nation has made a bold decision and the US strongly supports your request,” Bush told his counterpart Ukrainian President Viktor Yushchenko, two days before the NATO summit in Bucharest, Romania.

“My stop here should be a clear signal to everybody that I mean what I say: It’s in our interest for Ukraine to join,” he explained. A Membership Action Plan (MAP) outlines what a country needs to do to win an invitation for full NATO membership.

Russia is opposed to Ukraine and Georgia even starting the process, fearing a further loss of influence in two more of its Soviet-era Warsaw Pact neighbours.

Nine former Soviet bloc countries are already NATO members.