BELGRADE -- The emergence of an aggressive new communist leader in Serbia, Yugoslavia's largest republic, and an accompanying escalation of tension between ethnic Serbs and Albanians have increased fears here that nationalism in this key Balkan country is reaching explosive levels.

Slobodan Milosevic, who seized control of Serbia's powerful party organization in September, has alarmed politicians in Yugoslavia's five other republics by basing his political appeal on demands for stronger action to protect Serbian interests in the southern province of Kosovo, the site of frequent violent incidents this year.

Milosevic's drive, which resulted in the political downfall of one of Yugoslavia's relatively liberal leading communists, has helped raise tensions in Kosovo to the highest level since 1981 riots by Albanian nationalists led to military intervention in the province, journalists and diplomats here said. A special detachment of federal paramilitary police was dispatched to the area last month to quell activity by hostile Albanian and Serbian groups.

At the same time, the new Serbian leader has drawn strong criticism from journalists and political moderates by seeking to establish strong party control over Belgrade media and using them to advance his political cause. Some journalists charge that the openness and independence slowly developed by newspapers and television here in recent years is being reversed.

Milosevic's supporters deny that he holds hard-line nationalist views or that he seeks to control public debate. Nevertheless, other Yugoslav politicians and some diplomats say his tactics may provoke the rise of communists with nationalistic agendas in other republics or touch off violence in Kosovo.

"Whenever Serbia starts to stir, everyone else in Yugoslavia gets nervous," a western diplomat here observed. A Belgrade journalist added: "We are talking about a dangerous game of appealing to nationalist sentiments, a game that can easily be explosive."

Nationalism has always been the greatest internal threat to Yugoslavia, a country where six major national groups and several ethnic minorities live under a federal political system. With the death of postwar leader Tito in 1980, it lost a major unifying force.

During the 1980s, the sharpest nationalist conflict has been in Kosovo, a self-governing province of Serbia that is dominated by ethnic Albanians.

While Albanian groups frequently are charged with seeking to make Kosovo independent from Serbia or attempting to drive resident Serbs out of the province, Serbian groups have staged their own demonstrations in the last 18 months. Serbian radicals demand that Serbian authority, exercised from Belgrade, be fully restored in the region.

Although Kosovo has been a major focus of policy for the Serbian leadership in the past two years, Milosevic was the first politician who sought to exploit Serbian sentiment on the region, diplomats and politicians said. "He seized power on the basis of that one issue," a diplomat said. "It worked because that's the one issue every Serb is united on, party member or not."

The object of Milosevic's attack was his former political mentor, Serbian President Ivan Stambolic. Under his guidance, Milosevic, now 46, rose from vice president of a state gas company to president of a Belgrade bank in the 1970s, and from leader of Belgrade's communist organization in the 1980s to president in 1986 of the Serbian League of Communists.

Some government officials and diplomats say that Stambolic, who was known as an advocate of political and economic reforms, and Milosevic do not differ on these policy issues. However, other observers label Milosevic a conservative, favoring stronger central authority.

Several observers said Milosevic's sudden drive for power showed a flair and political agility rare among the lackluster politicians who have managed Yugoslavia since Tito's death. His chief instrument was the key Belgrade newspaper Politika, which under the direction of an ally of Milosevic began aggressive coverage of Kosovo -- focusing on alleged Albanian mistreatment of Serbs.

When Belgrade Communist chief Dragisa Pavlovic, an ally of Stambolic, called a party meeting to complain, Milosevic convened a party plenary session in late September at which Pavlovic and Stambolic were accused of failing to implement party policies on Kosovo.

With the overwhelming majority of the Serbian Central Committee behind him, Milosevic succeeded in having Pavlovic dismissed from his post, dealing a crushing political blow to Stambolic.

Since then, the new Serbian leader has forced the replacement of the chief executive of the publishing house that owns Politika and the editor of its influential sister weekly, Nin. Supporters say Milosevic now is seeking to install a new chief at Belgrade's state television station and is pressing for other media changes. News and television reports in Serbia now feature him.

Belgrade journalists said independent-minded colleagues were being reassigned or were resigning.

Milosevic's supporters deny a reassertion of central party power. "Some people are crying wolf," said Vladimir Stambuc, the Serbian party's executive secretary.