Bosnian Serbs prevented 160 British peacekeepers from reaching the eastern town of Gorazde on Tuesday and strengthening the U.N. contingent in the Muslim enclave.

The blockade reflected mounting tensions between the Serbs and U.N. troops, and the increasing willingness of Serbs to challenge the peacekeepers.In a series of recent skirmishes, Bosnian Serb forces have fired on U.N. troops. Danish tanks and Serb forces exchanged fire near Tuzla late Friday in a battle that the Serbs said killed nine civilians.

The Serbs reiterated charges that the peacekeepers had joined the fighting on the side of government troops, a claim the United Nations has denied.

French and Ukrainian reinforcements arrived in Gorazde on Monday. But the 160 British soldiers, sent to strengthen a total U.N. force of about 500, still were stuck in Rogatica, some 25 miles to the north, said U.N. spokesman Cmdr. Eric Chaperon.

Negotiations were under way for the release of the 51-vehicle convoy, Chaperon said in Sarajevo. "They can't return for the moment. They are stuck."

Asked if they were hostages, he said: "We have never used the word `hostages.' We're not going to use it today."

The British convoy has been at Rogatica since 2:30 a.m. Sunday. It had been blocked previously for several days at a Serb checkpoint just outside Sarajevo, about 35 miles to the west.

The chief U.N. diplomat for former Yugoslavia, Yasushi Akashi, met government leaders in Sarajevo on Tuesday and later was to see Serb leaders at their headquarters in nearby Pale, where he was expected to discuss the Rogatica impasse.

Chaperon said the situation was "generally calm" inside the Gorazde pocket, which came under Serb attack for three weeks before most Serbs were forced to withdraw.

In two air strikes last month, U.S. fighter jets under NATO's wing bombed Serb positions around the U.N.-declared "safe area" at the request of U.N. commanders on the ground.

NATO ultimatums forced the besieging Serbs to pull back heavy weapons 12.4 miles from Sarajevo in February and from Gorazde a week ago, but compliance has not been complete.