They say international peacekeepers have uncovered evidence of secret service involvement in fraud, money laundering and pornography - all aimed at upsetting the shaky Bosnian peace accord.

Croatia's refusal to cooperate with the international war crimes tribunal in the Hague, the diplomats warn, could result in the Balkan state being labelled a "rogue nation" like Yugoslavia. "Croatia is not as recalcitrant [as Serbia] but it is being difficult at the moment," one said.

The accusations come at a difficult time for Zagreb. President Franjo Tudjman is not expected to recover from stomach cancer - he is widely believed to be on a life support machine - and has not yet called parliamentary elections slated for next month. Opposition parties are reluctant to soften their stance towards war crimes investigations, fearing that Mr Tudjman's Croatian Democratic Union would portray any concession as a lack of patriotism.

Last month, after a year-long surveillance operation, French and Spanish troops raided a series of sites around the southern Bosnian town of Mostar and came away with thousands of documents, computer files and arms and explosives.

The "Operation West Star" haul is a political bombshell, western diplomats say, revealing the involvement of Croatia's five intelligence services in organised crime and activities inside the predominantly Croatian part of Bosnia aimed at preventing the return of ethnic Serb refugees, as demanded by the 1995 Dayton agreement.

If the pornography allegations were proved, they would be particularly embarrassing for the leadership. The ruling party is a pillar of the Roman Catholic church in Croatia.

"It's anti-Dayton and it's linked to the secret services," a spokesman for the Nato-led peacekeeping force said of the haul yesterday. "We will take more time to go through it."

Diplomats in Zagreb said the first chunk of evidence would be made public soon, but gave no timeframe. International officials trying to implement the Dayton pact have for years accused Zagreb of trying to carve out an "ethnically pure" area in southern Bosnia, similar to the Serb entity in the north and east. Bosnian Croats and Muslims form an uneasy federation that covers 51% of Bosnian territory.

Ivic Paselic, Mr Tudjman's adviser for internal affairs and head of the country's internal security forces, denied any Croatian government link to organised crime in Bosnia.

"According to our sources there is absolutely no ground for these accusations," he told the Guardian. "I know personally that Croatian intelligence have made a lot of effort to cooperate with western agencies to prevent crime."

Croatia is also under fire for its de facto refusal to extradite the former paramilitary leader Mladen "Tuta" Naletilic to the war crimes tribunal in the Hague, for blocking a tribunal investigation last week into a mass grave of ethnic Serbs, and for stopping officials investigating alleged war crimes during Croatia's 1995 offensives "Flash" and "Storm". The operations expelled Serb forces from the country and drove out hundreds of thousands of ethnic Serbs.

Mr Naletilic commanded a paramilitary group that operated around Mostar. He was arrested by Croatian police in 1997, charged with racketeering and incitement to murder, and has been in prison since then. Western officials say Croatia has strung out the extradition order on dubious medical grounds.

The war crimes tribunal will not reveal which mass grave it wants to investigate, saying it never tells anyone where its investigators are heading.

"Croatia refused to provide security for our investigation last week," was all spokesman Paul Risley would say. "Since we don't have jurisdiction over operations Flash and Storm either, Croatia's in danger of joining the league of rogue nations like Yugoslavia and Iraq."