The Western organisers of last weekend's elections in Bosnia have been accused by a coalition of ultra-nationalist Serb parties of preparing to falsify the results.
Diplomats fear the elections, in which Mrs Biljana Plavsic, the Western-backed president of the Serb-controlled half of Bosnia, appears to have lost power, could deliver the most serious blow to the Bosnian peace process since the US-mediated Dayton accord ended the civil war nearly three years ago.
"We all feel sick. It's a terrible blow," said one Western official in Bosnia. "We have to rethink our strategy."
The Serb accusations arose from the decision of the Organisation for Security and Co-operation in Europe (OSCE) on Tuesday to withhold preliminary election results.
According to diplomatic sources, Mr Nikola Poplasen, leader of the hardline Radical party, is heading for an unexpected victory over Mrs Plavsic.
No group was likely to win an outright majority in the Bosnian Serb parliament but diplomats said Mr Poplasen might be able to form a broader nationalist coalition and replace Mr Milorad Dodik, the pro-Western prime minister.
With Serb hardliners in power, and Muslim and Croat nationalists ruling the other half of Bosnia, the slow process of refugees returning to areas where they would be in an ethnic minority is likely to grind to a halt.
The rate of "minority returns", as the process is known, is seen by Western governments as the litmus test of the Dayton peace process. One diplomat predicted that almost all aid would be cut off from the Serb half of Bosnia and there would be mounting pressure, especially from the US, to reduce the 32,000 Nato-led troops based in the country.
The Radicals and their allies said they suspected the OSCE would manipulate the results. Mr Poplasen also accused the OSCE of threatening to strike him off the list of candidates for allegedly breaking the 24-hour period of pre-electoral silence.
The OSCE denied the results would be fixed. "We've created such a system that even if we wanted to it would be very difficult," a spokesman said.
The OSCE, which oversaw the $38-million presidential and legislative vote last Saturday and Sunday, expects to announce the outcome by the middle of next week.
Slowing down the vote count are 170,000 so-called tendered ballots, many of them cast by returned war refugees and those whose names were not on voter registration lists, that must be painstakingly verified one-by-one. Those ballots could be enough to swing some of the contests, analysts say.
In Vienna yesterday, the OSCE Bosnia mission chief, Mr Robert Barry, dismissed criticism of the way the elections were conducted, saying "computer breakdowns" affected only 2 to 3 per cent of the electorate.
Sarajevo's Oslobodjenje newspaper meanwhile criticised the OSCE, saying: "The problem is that here in Bosnia-Hercegovina, administrative and technical errors are taken and interpreted as political ones."
In another development, the OSCE's media experts commission has referred Serbia's RTS television to the Election Appeals Sub-Commission for airing an interview with the Serb hardliner, Mr Poplasen, when voting was underway on Saturday, in violation of the election rules. The Belgrade-based RTS is widely seen in Republika Srpska and is subject to Bosnian election laws.