Belgrade, Yugoslavia ? A power struggle in Yugoslavia deepened Saturday after President Vojislav Kostunica’s party announced it would boycott key ruling coalition meetings.

Kostunica’s Democratic Party of Serbia, a key member of the ruling alliance, the Democratic Opposition of Serbia, announced the boycott late Friday after refusing to endorse a draft law on cooperation with the U.N. war crimes tribunal in the Netherlands.

The party said it would withdraw from all meetings of Serbia’s presidency, which includes party leaders of all alliance members and shapes most Yugoslav policy.

In a statement, Kostunica’s party said it had “no intention of giving false legitimacy to the decisions passed by other parties.”

Dragan Marsicanin, the party’s deputy president, said Saturday that it had not left the coalition but only decided to boycott presidency meetings.

But Serbia’s Prime Minister, Zoran Djindjic, said the Democratic Party of Serbia had “abandoned” the coalition and warned that the 18-party alliance might push lawmakers from the Yugoslav president’s party out of the federal and Serbian parliaments.

Kostunica’s party holds 45 of 250 seats in the Serbian parliament, eight of the 130 seats in the federal parliament’s lower chamber and one of 40 in its upper chamber. Djindjic and his allies control 131 seats in the Serbian parliament.

The two are rivals and have clashed in the past over the U.N. tribunal that is prosecuting former Yugoslav political and military leaders accused of war crimes during the country’s violent breakup in the 1990s.

Djindjic, whose party is the largest in the coalition, supports the law on cooperation with the court in The Hague. He engineered the extradition of former President Slobodan Milosevic to the tribunal last year, while Kostunica fiercely opposed it.

Kostunica’s party’s rejection of the law led to the collapse of Yugoslavia’s federal government last year. Yugoslavia comprises two republics, the small Montenegro and the much larger Serbia.