DONETSK, Ukraine – The leader of a group of pro-Russian separatists said Friday that he would ignore an international agreement to de-escalate the political crisis in eastern Ukraine, saying his group would remain in the government buildings in the regional capital of Donetsk that it commandeered last weekend.

The agreement, announced Thursday by the United States, Russia, the European Union and Ukraine, called for all protesters to vacate the buildings they have occupied and lay down their arms.

But Denis Pushilin, the leader of the separatist group in Donetsk, which has declared an independent People’s Republic of Donetsk that no nation has recognized, told reporters that his followers would not comply until the interim government in Kiev resigned. He repeated his group’s demand for a referendum on the region’s future, similar to the one that preceded the annexation of Crimea by Russia last month.

Pushilin’s group is only one of many groups of pro-Russian militants that have seized buildings and arms in the east, and it was not immediately clear whether others would follow his line. But his rejection highlighted a critical omission in the Geneva agreement.

Russia “did not sign anything for us,” Pushilin said at a news conference in Donetsk.

There was no immediate official reaction from the Kremlin.

In an apparent attempt to help calm the crisis, Yulia V. Tymoshenko, the presidential candidate formerly imprisoned by the country’s ousted pro-Russian government, made a surprise visit to Donetsk on Friday. She told reporters at a news conference that she had come to “listen to the complaints of the demonstrators.”

It was unclear whether Pushilin would be receptive to Tymoshenko, a former prime minister who has opposed closer ties to Russia, an underlying tension in the crisis that has convulsed Ukraine for months.

Pushilin said he did not consider the new government in Kiev to be legitimate, and that if illegally occupied buildings are to be relinquished, then its officials, including the president, Oleksandr V. Turchynov, should vacate the administration building in the capital.

The government has not pulled forces back from a town north of here, Slovyansk, that was seized by pro-Russian separatists a week ago.

Tension on the ground continued to mount in the hours before the Geneva agreement was announced. Pro-Russian protesters tried to storm a Ukrainian base in the eastern city of Mariupol, prompting a firefight that left three of the activists dead, 13 wounded and 63 captured, according to Ukraine’s interim interior minister. In Donetsk, fliers appeared ordering Jews to register with the authorities.