BRUSSELS - NATO welcomed Albania and Croatia Wednesday as the 27th and 28th members of the world's biggest military alliance, after their ambassadors filed their instruments of accession with the US government.

"This is very welcome news," chief NATO spokesman James Appathurai told reporters in Brussels, where the North Atlantic Treaty Organisation has its headquarters.

"Albania and Croatia worked very hard to meet alliance standards, with regard to democracy, with regards to the state of their militaries," he said, two days before leaders of the Balkan states take their seats at NATO's table.

"They have overcome what was a difficult period in Balkans history not too long ago, to become contributors to regional stability and to international security through NATO already.

"Now they will take their full place in this alliance," he said.

Earlier, Albanian Ambassador Aleksander Sallabanda and Croatian Ambassador Kolinda Grabar-Kitarovic represented their countries at the ceremony in Washington hosted by US Deputy Secretary of State James Steinberg.

"We are determined to take our share of responsibility for global peace and stability in places such as Afghanistan but also in other parts of the world," Grabar-Kitarovic said.

Albanian Ambassador Aleksander Sallabanda gave a more general statement.

"As a NATO member country, Albania is ready to take up all the challenges and responsibilities that participation in the alliance entails," he said.

"In this context, we will work with responsibility and constructively with our partners to strengthen the ... alliance to protect our common values," he added.

In Zagreb President Stipe Mesic welcomed NATO's decision to admit Croatia and expressed hopes of his country joining the European Union.

"The Republic of Croatia has fulfilled one of its two foreign policy targets: to join the European Union and NATO," Mesic was quoted as saying by state-run HINA news agency.

"Today, the first of those goals has been achieved, we are becoming a full member of the NATO alliance and, by doing so, we have ensured security for our country," the Croatian president said.

"However, NATO is not only a military alliance but also protects democratic values, so by joining the alliance it is confirmed that Croatia has adopted and defends high democratic standards," he said.

Mesic also congratulated Albania.

Both countries already have troops in Afghanistan, where NATO is fighting its biggest and most complex mission trying to spread democracy throughout the insurgency-hit country.

Croatia and Albania were only able to join after Slovenia filed documents with the United States endorsing the two countries as members, a US official said Monday.

Slovenia took the action on Croatia's behalf at the State Department on Monday after doing the same for Albania on March 3.

Slovenia and Croatia are involved in a long-standing border dispute that has stalled Croatia's EU accession process. But Slovenia insisted that the row did not effect Zagreb's NATO bid.

After Greece ratified Croatia's accession in mid-February, Slovenia was the last of the 26 NATO members not to have handed the alliance its signature.

NATO leaders meet from Friday in Strasbourg, northern France and neighbouring Kehl in Germany for a two-day summit to focus on the alliance's future security challenges, but also its missions in Afghanistan and Kosovo.