"One, we have seen people attempting to survey specific camps, obviously," the department's spokesman, Ken Bacon, said. "Two, we have additional information that leads us to believe that there could be threats against American installations in Bosnia.
"So putting those two things together we think it's important to keep our security at the highest possible level. We are prepared for possible attacks." Mr Bacon would not say if a specific group in Bosnia was suspected of preparing to make attacks. He said the troops were on the same level of alert as US troops in Saudi Arabia have been since the 25 June truck bombing that killed 19 US servicemen.
Another US official said the heightened alert in Bosnia applied to all Nato peace-keeping troops, not just those of the US.
Earlier, as the Secretary of State, Warren Christopher, conferred with Nato civilian and military leaders in Brussels, US officials said the Bosnian elections in a month's time will fall far short of Western democratic standards. But the Nato Secretary General, Javier Solana, stressed how much had been achieved in the last eight months since the Nato-led peace Implementation Force (I-For) moved in.
A State Department spokesman, Nicholas Burns, said: "This is not going to look like a town in Germany or France or the US on election day. We're not going to create these conditions in the next 31 days."
The admission came as the commander of the Nato-led ground forces in Bosnia, Lieutenant-General Sir Michael Walker, helped defuse a confrontation with the Bosnian Serbs who had refused to allow Nato troops to inspect part of a heavy weapons storage site.
General Joulwan and Dr Solana visited the Bosnian Serb capital of Pale last Monday to ensure that the site could be inspected. But it took time for the agreement to filter down to the commanders on the ground.
Early yesterday morning General Walker took the new Bosnian Serb President, Biljana Plavsic, to the arms site at Han Pijesak, 30 miles east of Sarajevo, where the Bosnian Serbs had refused to allow Nato inspection teams access on Monday. Nato officials said the move was, in part, to test the influence of Dr Plavsic, who replaced indicted war crimes suspect Radovan Karadzic as President. General Walker returned to Sarajevo yesterday after completing the inspection.
In response to Monday's stand-off I-For immediately implemented a plan codenamed "Fear Naught", withdrawing troops from vulnerable positions in Bosnian Serb territory and trying to extract foreign civilians such as aid workers. In the other half of Bosnia, controlled by the Muslim- Croat federation, there were reports of US forces intercepting a letter indicating that Iranian terrorists planned to attack Nato positions, but officers in Sarajevo said they were unaware of such a threat.
Mr Christopher met Dr Sol-ana and General George Joulwan, Nato's Supreme Allied Commander Europe, in Brussels, to discuss I-For's support for the elections which aim to create common institutions for all Bosnia. Mr Burns said Mr Christopher will discuss this with the presidents of Bosnia, Serbia and Croatia in Geneva today.Reuse content