By nightfall Canadian troops with NATO's stabilization force (SFOR) were enforcing an uneasy peace in Drvar, a town of 17,000 where tension between Serbs and Croats has been bubbling for weeks.

Fourteen people were injured, including Drvar's Serb mayor Mile Marceta, a local police official told AFP by telephone.

``The situation is calming down,'' the official added.

Bosnian television showed video of SFOR troops, in full battle gear and automatic rifles over their chests, holding back restive civilians as armoured personnel carriers prowled deserted streets.

Reporters who flew over Drvar in a UN helicopter saw several fires and the burned-out remains of the local station of the UN's International Police Task Force (IPTF), whichoversees all police departments in Bosnia.

``We don't know the precise reasons for this violence,'' said IPTF spokesman Alun Roberts in Banja Luka, the base for international operations in Bosnia's northwest and West.

But tension has been growing between Croats and Serbs in the area, starting in Drvar with a string of house-burnings and, on April 16, the murder of an elderly Serb couple who had come to reclaim their pre-war home.

Outraged by the murders, the United Nations and the international community's high representative in Bosnia, Carlos Westendorp, sacked Drvar's deputy mayor and police chief -- both Croats.

On Thursday, one week after the double murder, 1,000 Serbs trapped Sarajevo's Cardinal Vinko Puljic for six hours inside a church in Derventa after he went to the northern town for a Saint George's Day mass.

Ironically, Puljic appealed on Friday for Croats -- who are Roman Catholics -- not to react violently, but he also announced that another mass would be held on Saturday in theDerventa area, attended this time by Bishop Pero Sudar. Besides the IPTF post, at least five buildings in Drvar were hit by fire, as well as several cars belonging to international organizations, Roberts said. IPTF officers were also stoned as they were being evacuated. Also attacked was the municipal office building, where a meeting of the local election Commission was under way. Fire was set to it and its windows were smashed by stones.

Vladamir Soljic, the Croat vice-president of Bosnia's Muslim-Croat entity, said Thursday's incident at the church in Derventa was ``very likely'' the spark that ignited on Friday's riot.

``But the causes are much deeper,'' he said on Croatian television.

``Basically, it results from the massive return of Serbs to Drvar when nothing is being done for Croat (refugees now in Drvar) to return to their homes.''

Drvar was 97 per cent Serb before the war, but now is dominated by Croats forced to move there by fighting in other parts of the country. International agenciessay only 1,500 Serbs now live there.

In a late-night statement, Westendorp's office called the incidents on Thursday and Friday ``appalling acts of organized political violence'' that were ``totally unacceptable.'' ``The international community stands by those who wish to return to their homes in Drvar,'' it added. ``We shall not permit today's thuggery to stand in the way of that aim.''