PRISTINA, Serbia, Dec 4 (Reuters) - International truce monitors in Kosovo expressed concern on Friday over attacks in the restive province which ethnic Albanians have sworn to avenge.

Unclaimed attacks and a shoot-out between Yugoslav border guards and armed men crossing from Albania left 12 dead and two injured on Wednesday and Thursday. It was the highest casualty figure for weeks in the troubled south Serbian province.

The OSCE (Organisation for Cooperation and Security in Europe) which is building a 2,000-strong team to assess compliance with a U.S.-brokered truce in Kosovo, issued a cautious statement in the wake of the violence.

"It is hard to see that they (the incidents) are in any way related or represent a dramatic change in the security situation in the short term," said the OSCE, noting that both Serbs and ethnic Albanians were breaching the ceasefire.

"The OSCE condemns violence from any quarter at this delicate stage in the search for a political solution," it said.

Western diplomats in the regional capital Pristina said the mission's worries went further than expressed in the mildly-worded official release, particularly over the shooting of a junior seperatist guerrilla leader on Wednesday.

Representativies of the Kosovo Liberation Army, which is fighting for independence for the predominantly ethnic Albanian province, said the killing of Hyrzi Tallaj was an assassination by Serbian security forces.

"That is destabilising," said one Western envoy. "It appears to be the work of a special police unit. They can no longer roll tanks into villages so they are having to operate their counter-insurgency campaign more discreetly."

Tallaj was killed along with two friends when his car was sprayed with bullets by unknown gunmen in a Pristina side street. He was buried on Thursday with KLA military honours.

"The KLA will avenge the death of these three martyrs," the leading Albanian daily Koha Ditore reported Valon Murati, a KLA officer, as telling mourners by Tallaj's grave.

"The occupier (Serbia) is gaining from the armistice by eliminating our best people," it reported Murati saying.

Serbian-dominated Yugoslavia halted a fierce offensive on ethnic Albanians in October under international pressure, after months of fighting that left at least 1,500 dead and forced a quarter of a million from their homes.

The West is trying to keep the peace in the region long enough for it negotiate a deal on autonomy for Kosovo. But neither Serbs nor ethnic Albanians are enthusiastic and each is waiting for the other to provoke a return to violence.

An ethnic Albanian was killed and a Serb motorist wounded in separate attacks on vehicles in contested territory west of Pristina on Wednesday evening and Thursday morning.

Nobody has claimed responsibility for the mysterious incidents. Each side has accused the other of trying to implicate the enemy, and tension in the area has increased.

In the deadliest recent incident Yugoslav army soldiers killed eight ethnic Albanians in a fire-fight along the border. It was the third clash between border guards and infiltrators in a week in which Belgrade had already protested to Albania.

Western diplomatic sources said the eight dead appeared to be KLA guerrillas, adding that the KLA was trying to rearm via supply routes over the mountains into neighbouring Albania.

The source said there was little the OSCE could do about an increase in violence that falls short of full-scale war.

"All we can do is report," he said. "Cynically speaking, such incidents are not going to create a humanitarian crisis and that's the bottom line for us."