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.