PRISTINA, Yugoslavia (AP) _ More international monitors headed into Kosovo on Tuesday to shore up a fragile truce following the worst outbreak of fighting between Serb forces and ethnic Albanian rebels in months.

NATO, meanwhile, repeated warnings that it stands ready to intervene if the two sides fail to respect the Oct. 12 agreement between Yugoslav President Slobodan Milosevic and U.S. envoy Richard Holbrooke, which ended seven months of fighting and averted NATO airstrikes.

NATO Secretary-General Javier Solana, speaking in Brussels, Belgium, cautioned both sides ``not to endanger the fragile security situation.''

The October agreement was severely strained by four days of fighting that began Christmas Eve between government forces and the ethnic Albanian Kosovo Liberation Army. At least 15 people died. Kosovo, a province of Yugoslavia's main republic of Serbia, was generally quiet Tuesday.

But the ethnic Albanian-run Kosovo Information Center, reported some shooting late Monday and early Tuesday on the outskirts of Kosovska Mitrovica, 20 miles northwest of the provincial capital of Pristina, which ``made the situation there very tense and difficult.''

International and Serb sources, speaking on condition of anonymity, said five bodies were found Tuesday in widely scattered areas of Kosovo, including two in Kosovska Mitrovica.

The Organization for Security and Cooperation in Europe, which already has some 600 monitors in Kosovo, said an uncertain number of American, Canadian and European observers arrived in Kosovo late Tuesday. The rest were expected in a day or two, bringing the total of new arrivals to about 100. The OSCE expects to have a full compliment of 2,000 verifiers by mid-January, according to spokesman Sandy Blyth.

Verifiers, however, do not carry weapons, and critics among ethnic Albanians and Serbs have questioned whether the mission will be able to prevent a new round of fighting, possibly in the spring.

Rhetoric on both sides underscores the bitterness between Serbs and the ethnic Albanians who demand independence for the province.

Rebel spokesman Adem Demaci said the guerrillas will respect the cease-fire ``but we will defend ourselves with all means, if necessary.''

Yugoslavia's foreign minister, Zivadin Jovanovic, demanded that the KLA be declared a terrorist organization. And the ultra-nationalist Serbian Radical Party, a close ally of Milosevic, called Tuesday for ``complete extinction of Albanian terrorist gangs,'' as the only way to solve the Kosovo crisis.