The leaders of Bosnia's main Muslim, Croat and Serb parties gave their support on Monday to a decentralisation plan for the ethnically divided country that fought a civil war from 1992-95.

Details of the plan, which would be enacted as a revision to the constitution, remained sketchy, but it calls for a division of the country into four administrative units, officials said.

It was unclear how this would fit with Bosnia's efforts to join the European Union (EU), which wants the Balkan country to strengthen its central state before it joins the 27-member bloc.

"We agreed that Bosnia should be a decentralised state with three layers of government of which the second one is divided into four territorial units each with their own legislative, executive and judicial powers," the party leaders said in a joint statement.

The leaders did not elaborate but said a concrete plan for the new organisation will be presented at the meeting of the Party of Democratic Action (SDA), Alliance of Independent Social Democrats (SNSD) and Croatian Democratic Union (HDZ) next month.

The office of the international community's top peace overseer in Bosnia declined to comment on the agreement.

Efforts to revamp Bosnia's constitution, enshrined in the Dayton peace treaty that ended the bloodiest conflict in Europe since World War Two, and seen as a key condition for joining the European Union, have been in the works for two years.

Inter-ethnic squabbling has hampered progress to reform the charter to take away power from Bosnia's autonomous entities, the Serb Republic and the Muslim-Croat federation, which have coexisted in an uneasy alliance since the end of the war.

Officials say revising the constitution is likely to be a lengthy process. Lawmakers must first debate and accept the request by the SDA, SNSD and HDZ before a constitutional commission can be formed to draft a new charter.

The second largest Bosnian Muslim faction, the Party for Bosnia-Herzegovina (SBIH), said it would back a split of the country into four administrative units only if they are based on multi-ethnic, economic, geographic or similar criteria.

"However, if the initiative goes in favour of the creation of the predominantly Croat entity and the Sarajevo district, it would only serve as a disguise for the division of the country on territorial and ethnic principles," SBIH said in a statement.

The Bosnian capital Sarajevo was under siege from Serb forces for 43 months during the civil war.

Bosnia signed a pact with the EU last June, a first step towards eventual membership nearly 13 years after the war's end.