The Clinton administration has summoned Croatia's President, Franjo Tudjman, and his foreign and defence ministers to Washington to spell out that Croatia will pay a price if it continues to undermine key provisions of last year's Dayton peace settlement for Bosnia.
"We've already told the Croats we're not going to let them get away with this boycott, that their behaviour in Bosnia will shape their future ties with the West, on whom they must rely for economic development," a senior US official said. "We've told Tudjman that if the [Muslim-Croat] federation falls apart and he is the reason, if he makes any attempt to annex part of Bosnia, Croatia will be an international pariah."
US negotiators brokered a deal on Wednesday under which Bosnian Croat leaders committed themselves to abolish once and for all their state of Herzeg-Bosnia. The rogue entity, a Croat mirror image of the Bosnian Serb republic, should have been dissolved under the Dayton terms, but it has survived with thinly-disguised support from Mr Tudjman.
Since even the most solemn promises often turn out to mean nothing in former Yugoslavia, US officials suspect that the Bosnian Croats will not honour their latest commitment unless Mr Tudjman comes under pressure to bring them into line.
Croat nationalists established Herzeg-Bosnia in 1992, and since then the territory has been little more than an appendage of Croatia. It uses the Croatian flag and currency, and its ruling party is a satellite of Mr Tudjman's Croatian Democratic Union (HDZ).
Under the deal brokered by John Kornblum, the US mediator, Bosnian Muslim and Bosnian Croat leaders have until 8 August to set up power-sharing institutions for the Muslim-Croat federation, which occupies 51 per cent of Bosnia.
These institutions, designed to prevent a three-way partition of Bosnia into Muslim, Croat and Serb areas, are to start operating after Bosnia's first post-war general elections on 14 September.
Despite the apparent progress on abolishing Herzeg-Bosnia, US negotiators are finding it difficult to extract a pledge from Mr Tudjman and his Bosnian Croat clients to end the division of Mostar. The city is split into a Muslim-held eastern sector and a western sector that US and European officials say is under the control of Croat nationalists, war profiteers and gangsters.
The Bosnian Croats, who regard Mostar as the capital of Herzeg-Bosnia, narrowly lost municipal elections to their Muslim rivals on 30 June. They have refused to recognise the results and take up their seats in the city council, thereby perpetuating Mostar's division and keeping alive their hopes of maintaining a distinct Croat political unit in Bosnia.
US officials discussed the Mostar crisis with Mr Tudjman in Zagreb on Wednesday and found him in uncompromising mood. However, he is likely to be more conciliatory with Mr Clinton as he attaches great importance to the US-Croatian relationship.
The EU has administered Mostar since 1994 but has made no progress towards ending its division. The present administrator, Sir Martin Garrod, has told the Bosnian Croats that the EU will pull out on Sunday unless they join the city council, but they greeted his warning with scorn.
Western governments regard it as essential that the Croats recognise the Mostar election results and end their council boycott if next month's all-Bosnian elections are to look credible.
Bosnia's Muslim-led government contends that the polls will not be free and fair because both Bosnian Serbs and Bosnian Croats are hostile to the central purpose of the elections. That is to uphold the principle that Bosnia is a single state, even if split into Muslim-Croat and Serb zones.Reuse content