Denis Pushilin of the self-appointed Donetsk People's Republic told reporters the insurgents do not recognize the Ukrainian government as legitimate.
Ukraine and Russia on Thursday agreed in Geneva to take tentative steps toward calming tensions along their shared border after weeks of conflict since Ukraine's former leader fled to Russia in February and Russia annexed Crimea in March. The deal calls for disarming all paramilitary groups and the immediate return of all government buildings seized across the country.
But Pushilin, speaking at the insurgent-occupied regional headquarters in the eastern city of Donetsk, said the deal specifies that all illegally seized buildings should be vacated. He said Ukraine's new interim government in the capital, Kiev, is also occupying public buildings illegally.
"This is a reasonable agreement but everyone should vacate the buildings and that includes (Arseniy) Yatsenyuk and (Oleksandr) Turchynov," he said referring to the acting Ukrainian prime minister and president.
Ukraine has scheduled a presidential election for May 25, but Pushilin reiterated a call to hold a referendum on self-determination for the Donetsk region by May 11.
Ukraine has faced months of turmoil, first in Kiev by protesters angry that former President Viktor Yanukovych wanted closer ties with Russia instead of Europe, then in eastern Ukraine, by those who want better ties with Russia. Now many of the buildings in the east occupied by the tacitly Moscow-supported insurgents are in the hands of highly trained gunmen, a situation that has complicated authorities' plans to retake them.
Pushilin said the insurgents would not handover their weapons until the government halts efforts to reclaim the occupied buildings.
"As far disarmament goes, the Kiev junta has already begun violating its agreements since yesterday, by announcing that it will not pull its troops out of Slovyansk and Kramatorsk," Pushilin said, referring to two cities occupied by the insurgency.
Despite announcing a security operation with great fanfare, the government in Kiev has taken few practical measures to reclaim its buildings.
Kiev-based political analyst Vasim Karasyov said Ukraine's fledgling government does not have the resources to resolve the standoff in eastern Ukraine militarily, so it's going to have to negotiate with the pro-Russian protesters.
"(Kiev) should finally listen to the demands of those people," he said. "They don't even know what their demands are. Maybe they are reasonable. The government in Kiev is pretending that there are no problems in the east."
Prime Minister Arseniy Yatsenyuk told parliament Friday that the government has drafted a law to offer amnesty to all those willing to lay down their arms and leave occupied government buildings.
In Washington, President Barack Obama conveyed skepticism late Thursday about Russian promises to de-escalate the volatile situation in Ukraine, and said the United State and its allies were ready to impose more sanctions if Moscow doesn't make good on its commitments.
"My hope is we do see follow-through," Obama said.
Russian Foreign Minister Sergey Lavrov on Thursday emphasized that the requirement to abandon occupied building applied to all parties. But although Russia has declined to recognize the legitimacy of Ukraine's interim government, it has not said they should vacate their offices.
Instead, the reference appears to be to groups such as the ultranationalist Right Sector movement, whose activists are occupying Kiev city hall and a Kiev cultural center.
The barricade-lined space in front of the Donetsk regional administration building, a mustering point for pro-Russian supporters, was almost totally empty Friday despite the warm weather. Patriotic Soviet-era music blared over loudspeakers as a few dozen, mainly elderly, people sat around chatting.
One man in the square, 56-year-old militia member Igor Samoilov, said he would not support pulling back from any seized buildings.
"Russia can play these games with the West, but we will not," said Samoilov.
Sitting nearby, 86-year-old Yuri Kovalchuk said Moscow needed to intervene directly to settle matters.
"Peace will only prevail when the Kremlin will bring in its troops. As it did in Crimea," he said.
Russia's Foreign Ministry had no immediate comment on Pushilin's statement. But pro-Russian Ukrainian presidential candidate Oleh Tsaryov, whose statements often echo Moscow's stance, was quoted by the Russian state news agency RIA Novosti on Friday as saying Right Sector activists should be the first to lay down their arms.
Vasilyeva reported from Kiev. Yuras Karmanau in Donetsk and Laura Mills in Moscow also contributed to this report.