Denis Pushilin of the self-appointed Donetsk People's Republic told reporters the insurgents do not recognise the Ukrainian government as legitimate.
Ukraine and Russia agreed yesterday 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.
"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.
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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. Such a referendum in Crimea led to its annexation by Russia.
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 pro-Russian supporters.
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.
In a sign that Ukraine's fledging government is ready to meet some of the protesters' demands, acting President Oleksandr Turchynov and Prime Minister Arseniy Yatsenyuk issued a joint statement today saying the Ukrainian government is "ready to conduct a comprehensive constitutional reform that will secure powers of the regions," giving them a greater say in local governance.