Nato leaders meeting in Prague have invited seven more countries to join the alliance in what will be the biggest expansion in its history.

The candidates - Bulgaria, Romania, Estonia, Latvia, Lithuania, Slovakia and Slovenia - are due to become full members in 2004 swelling the alliance's membership to 26.

In what he called a defining moment, Nato Secretary General George Robertson said a "transformed Nato will be a stronger, more effective partner for all our friends" - even though many of the new members are bringing with them only small and poorly equipped armies.

US President George W Bush warned that all Nato members - both new and old - must contribute military strength to the alliance, even if it means increasing defence spending.

Invited countries

Bulgaria Estonia Latvia Lithuania Romania Slovakia Slovenia

The BBC's Europe correspondent, Chris Morris, says that although the enlargement will be trumpeted as a sign of security and democracy in Europe, to some, it will be seen as proof that Nato is less a top-notch military alliance and more just a political club.

The Prague summit will also address reforming the organisation to deal with terrorism and to speed up Nato's response to conflicts.

The discussions will be overshadowed by debate about Iraq and by tough talk from President Bush that deception from Saddam Hussein will not be tolerated.

Mr Bush is expected to use the gathering to gauge other alliance members' willingness to use military force against Iraq.

Warm welcome

In their opening statements, however, leaders focused on the enlargement process.

"Aspirant countries have been working hard to modernise and reform," Lord Robertson said. "It is a reflection of their political determination to join Nato that they have met this challenge. We have been working to ensure that Nato itself is ready to enlarge."

An enlarged alliance will have 26 members

Mr Bush said accepting new members "will not only add to our military capability, it will refresh the spirit of this great democratic alliance. It reinforces the commitment to a Europe which is free and at peace".

He aimed to soothe Russian concerns about its neighbours becoming members of the organisation which once opposed the Soviet Union in the Cold War. Three former Soviet republics - Estonia, Latvia and Lithuania - were invited to join Nato on Thursday.

"A larger Nato is good for Russia as well... [Russia] too will gain from the stability and security of nations to its west," he said.

He stressed the United States' commitment to Nato - which some have doubted under the Bush administration - describing the alliance as his country's "most important global relationship".

Bush's priorities

Mr Bush said that as Nato expanded, the organisation had to develop new capabilities - and that would mean all countries had to make their defence spending more effective.

US fighters are boosting security for the summit

He said that Nato's options when forces were needed quickly in Afghanistan had been "limited" and said a highly equipped special response force constantly ready for deployment should be set up.

"Every member must make a military contribution to this alliance," Mr Bush said.

"Never has our need for collective defence been more urgent," he said.

He highlighted several capabilities which Nato particularly required, including:

• Being better able to fight side-by-side
  
• Being more mobile and able to be more swiftly deployed
  
• Having more special operations forces and better precision strike capabilities
  
• Having a more modern command structure

Mr Bush said efforts to set up a rapid response force consisting of highly equipped air, ground and sea forces should begin at the Prague summit.

The unit would comprise 21,000 combat-ready troops able to be deployed quickly to trouble spots around the world and play a key role in the US-led war on terror. It could be operational by 2004.