SARAJEVO -- Serb troops and rebel Moslem militia made big territorial gains in an offensive on the Bihac enclave in Bosnia as heavily armed British troops dug into positions near Sarajevo, UN officials said Monday.


Coordinated action by Croatian Serb and Bosnian Serb troops together with rebel Moslems appeared intent on bisecting the Bihac enclave, a UN-declared "safe area," in attacks from the north and west since last Wednesday, a UN spokesman said.


"This is an extraordinarily escalatory step. We have an attack across an international border, coordinated among three factions," Alexander Ivanko, a UN spokesman in Sarajevo, said.


He added that Croatian Serbs may have gained control of up to 75 square kilometers of the pocket.


The Bihac crisis could explode into a wider war in former Yugoslavia as Croatian government troops are poised along UN truce lines near the Adriatic to attack the rebel Krajina Serbs in Croatia if the Bosnian enclave looks like falling.


The United Nations went on alert for Croatian army action after Zagreb on Saturday accepted a Bosnian government call for urgent military assistance in Bihac.


As Bihac suffered fresh assaults, British troops deployed on Mount Igman early Monday to defend UN peacekeepers in nearby Sarajevo.


In addition to two British artillery batteries comprising 200 men and 12 105-millimeter light guns and a company of 120 infantrymen on Igman, 500 French Foreign Legionnaires were heading toward Sarajevo from their base near the Bosnian town of Tomislavgrad, a UN spokesman said.


The British and French units, part of the UN Rapid-Reaction Force, or RRF, were sent to the area after Bosnian Serbs killed two French peacekeepers Saturday.


Immediately after their arrival, the British RRF artillery and infantry units began digging gun emplacements. They are there to help ensure that a track leading to the city remains open for humanitarian supplies.


UN spokesman Lieutenant Colonel Chris Vernon said the arrival of the British 105-millimeter guns provided the UN with a more serious deterrent than anything the Bosnian Serbs had faced from the UN mission so far.


But he emphasized that the guns were deployed as a defensive measure.


"You hit us and we're going to hit you with slightly heavier stuff than we've had before. That's the message.".


The deployment followed a renewed warning to Bosnian Serbs from Western powers to expect the hardest air strikes yet if they attacked the UN-declared "safe area" of Gorazde.


The warning was made in a joint statement issued Sunday by the United States, France and Britain.


In Washington, the White House said the Bosnian Serbs were told "that if military action is undertaken against Gorazde, substantial air actions will be mounted. If necessary, these actions will be at unprecedented levels."


Senior military officers from the three countries had delivered the warning to the Bosnian Serb army commander, General Ratko Mladic, during a meeting in Belgrade, diplomats said.


NATO ambassadors were meeting in Brussels to put meat on the bones of an earlier air-strike threat to the Bosnian Serbs issued after a 16-nation crisis meeting on Bosnia in London on Friday.


That declaration, not supported by Russia, immediately created confusion by singling out Gorazde and ignoring the other so-called "safe areas" such as Bihac and Zepa.


Vernon said reports from Ukrainian peacekeepers in besieged Zepa indicated fighting had died down but the enclave remained tense on Monday.


The London conference was convened after Bosnian Serbs captured the nearby "safe area" of Srebrenica on July 11, detaining Moslem males and forcing some 30,000 Moslems to flee.