Washington/Brussels - The North Atlantic Treaty Organization (NATO) on Wednesday formally welcomed Albania and Croatia as fully fledged allies, raising the total number of member states to 28.

"Today, Albania and Croatia have completed the accession process, and have joined the Alliance as members," NATO Secretary General Jaap de Hoop Scheffer said in a statement.

"I warmly congratulate them on this historic achievement. In becoming NATO members, Albania and Croatia share the benefits and responsibilities of collective security," the alliance's chief said.

The statement came shortly after the ambassadors of the two Central European countries handed over the "instruments of accession" during a ceremony in Washington. The bureaucratic act completed a year-long process involving the ratification of their accession by existing NATO allies.

The move allows Albanian Prime Minister Sali Berisha and Croatian Prime Minister Ivo Sanader to join US President Barack Obama and other NATO leaders at the alliance's 60th anniversary summit in Strasbourg, Baden Baden and Kehl on Friday and Saturday.

A flag-raising ceremony marking their accession is to be held at NATO headquarters in Brussels on April 7th.

"Albania and Croatia have worked very hard to meet alliance standards with regards to democracy and the state of their military," NATO spokesman James Appathurai said earlier in Brussels.

The two countries, which are also aspiring to become European Union member, received a formal invitation to join NATO at last year's summit in Bucharest.

"We've reached a goal we have been striving for 20 years," Croatian Prime Minister Ivo Sanader said.

Albanian Defence Minister Gazmend Oketa said his country's membership would be "a positive card which will help Albania to more rapidly approach the European Union."

"We don't consider membership in NATO as a final station but as a constant work process with duties and responsibilities," Oketa told the Albanian news agency ATA.

Croatia's accession had been threatened until only last week by a prolonged border dispute with NATO member Slovenia. The issue was finally resolved on Friday, when Slovenian President Danilo Turk signed a law paving the way for its accession.

Both Croatia and Slovenia emerged as independent countries from former Yugoslavia in 1991.

Speaking in Brussels, Appathurai said Croatia and Albania had both "overcome what was a difficult period in Balkan history."

Albania and Croatia are already contributing to NATO's International Security Assistance Force (ISAF) mission in Afghanistan, with 140 and 530 soldiers respectively.

Another member of the former Yugoslavia, Macedonia, has been prevented from joining the transatlantic alliance because of a name dispute with neighbouring Greece. (dpa)