Volker was speaking before the House Committee on Europe on US policy towards NATO and the forthcoming alliance summit in Riga, Latvia.
The aim of the Riga summit is to prepare NATO for new post-Cold War challenges, while the next summit, due to take place in early 2008, is expected to focus on NATO's enlargement.
All three countries should make further progress in reform in order to be ready by 2008 and the US is intensely helping them build the strongest possible candidacy for NATO membership by that time, Volker said.
Volker, who visited Croatia last month to assess its readiness for NATO, told Hina that Croatia had made great progress in political, economic and defence reforms, but stressed that it should continue work on establishing an independent and well-functioning judiciary and curbing corruption.
On the subject of defence, Volker said that Croatia should continue transforming its large army into an expedition-type armed force trained and capable of operating from great distances together with NATO forces.
The US would like to see stronger support of the Croatian public for NATO membership, because it is important that the country wants to be a member of the alliance and to enjoy public support for participation in NATO operations, the State Department official said.
Some 30 per cent of Croats currently support their country's NATO membership bid.
Daniel Fata, Deputy Assistant Secretary of Defence for European and NATO Policy, said that the three Adriatic Charter member states still have a lot of work to do to modernise their armed forces and reduce personnel costs to the benefit of operational and other needs, in which he said they were being assisted by the US Defence Department.