By TOM COHEN Associated Press Writer

PRISTINA, Yugoslavia (AP) - Three Kosovo Serbs were injured by a grenade in the latest ethnic violence to undermine the international peacekeeping effort in the war-torn province, NATO said today.

The grenade exploded late Tuesday in Gnjilane, the largest town in the southeastern region where American troops are supposed to keep order. The three suffered minor injuries and no details were available about the assailants.

The American sector has one of the highest concentration of Serbs left in the province. On Monday, nine mortars were fired at a Serb village in the U.S. zone, killing two Serbs and injuring five others.

The Yugoslav government, in a note to U.N. and NATO officials, demanded ``urgent measures to provide security for Serbs'' in the province, the state-run Yugoslav news agency Tanjug said today.

The note, addressed to British Gen. Mike Jackson, head of the peacekeeping force, and U.N. chief of mission Bernard Kouchner, said that despite the presence of international peacekeepers, ``the Serb and non-Albanian population of Kosovo is constantly exposed to the terror of the Kosovo Liberation Army.''

Kouchner is the top civilian administrator in Kosovo, heading the effort to rebuild the province and create a multiethnic government. Kouchner addressed staff at Pristina's main hospital today, telling them the U.N. administration would pay the salaries of 2,000 professionals.

The ultranationalist Serbian deputy premier and leader of the Radical Party, Vojislav Seselj, claimed in a television interview Tuesday that conditions for Kosovo Serbs were worst in the American sector. But he urged Serbs to stay. Threats by ethnic Albanians have prompted most of the province's 200,000 Serbs to flee.

On Tuesday, Serb villagers in Klokot buried the victims of the mortar attack - a 23-year-old woman and a 24-year-old man. An American tank stood guard outside the Serbian Orthodox church as the solemn procession passed through the village.

In a statement today, the NATO command condemned ethnic attacks.

``We will, however, only succeed in generating a secure environment with the full cooperation of the local population,'' the statement said. It welcomed a recent call by a KLA commander, Agim Ceku, ordering guerrillas to respect human rights.

Meanwhile, peacekeepers seized rifles and ammunition in two separate raids in Kosovo, NATO said. German troops seized rifles, pistols and forged ID cards in a raid on a cafe Tuesday in Prizren, while Italian troops confiscated nearly 50 rifles and 250 rounds of ammunition in a pre-dawn raid today on a house in Djakovica.

Russia, which has about 3,600 troops in Kosovo, called on its NATO peacekeeping partners to take ``urgent actions'' to stop anti-Serb attacks in the province.

``Violence is raging in Kosovo. The killings of peaceful civilians, arsons and robberies of non-Albanians multiply,'' the Russian Foreign Ministry said Tuesday. ``In essence, ethnic cleansing is in full swing in Kosovo.''

A recent NATO success in containing ethnic tensions appeared threatened when a Serb leader in the divided town of Kosovska Mitrovica denied Tuesday he had agreed to a NATO-U.N. plan to resettle ethnic Albanians in their homes in the Serb-majority enclave of the city.

The Serb official, Oliver Ivanovic, cited ``relentless Albanian onslaughts'' for the refusal of the deal, Tanjug reported.

Ethnic Albanians have clashed frequently with French troops barring them from crossing a bridge to the mainly Serb side of the city.

But on Monday the deal was struck to allow 25 ethnic Albanian families a day to move back to homes they lost on the Serb side. U.N. officials had said the city's Serb leaders had given ``tacit approval.''