International attempts to de-escalate tensions in Ukraine were floundering on Friday as separatist groups in the east declared that they had no intention of leaving occupied buildings and accused Kiev of violating an agreement reached in Geneva on Thursday.

Russia, Ukraine, the EU and the United States struck a diplomatic deal in the Swiss city, following seven hours of talks, that was supposed to see illegal groups withdraw from municipal buildings and hand in their weapons.

Twenty-four hours later there were no signs that any of the anti-government groups were preparing to budge. Instead, protest leaders said they would continue their occupations until their demands were met. A rebel militia seized an administration building in Seversk, a small town outside the regional capital Donetsk.

At a press conference on Friday Denis Pushilin, the self-styled leader of the "Donetsk People's Republic", said his supporters would stay put until a referendum on the region's future status was held. He dismissed the current pro-western government in Kiev as illegitimate. "We will continue our activity," he declared.

Pushilin said no meaningful de-escalation was possible while Ukraine's interim prime minister Arseniy Yatsenyuk and president Olexsandr Turchynov were still in their jobs. "We understand that everyone has to leave buildings or nobody does. Yatsenyuk and Turchynov should vacate theirs first," he said.

Moscow's envoy to the European Union reiterated this position, telling Russian state television that authorities in Kiev had "incorrectly interpreted" the Geneva deal. He said Ukraine's new leadership mistakenly believed that the deal "only applies to the eastern and southern provinces" when it also applied to "the ongoing occupation of Maidan [Independence Square in Kiev]".

Pro-Russian separatists grabbed a string of public buildings across eastern Ukraine a week ago. The militia units – some of them similar to the armed "little green men" who appeared in Crimea in February – have occupied them ever since. Nato says the separatists include professionally trained undercover Russian soldiers. Moscow denies this.

In Kiev, Ukraine's acting foreign minister Andriy Deshchytsia said the next few days would demonstrate whether Russia actually intended to implement the Geneva deal, signed by Russia's foreign minister Sergei Lavrov. "I don't know Russia's intentions. But minister Lavrov did promise that they want to de-escalate. So we will see in a few days if it was [a] sincere promise and sincere participation."

The separatists, however, seem in little mood to give ground. Pushilin said Kiev had already violated the Geneva accord by refusing to pull its military units from the east of Ukraine. "They have not withdrawn their forces out from Slavyansk," he said. Beleaguered Ukrainian troops occupy a rustic aerodrome close to Slavyansk, north of Donetsk, and neighbouring Kramatorsk. On Wednesday they suffered the ultimate humiliation when armed separatists, seemingly led by Russian officers, seized six armoured vehicles from them and drove off.

Pushilin delivered his anti-Kiev message to Russian state television, which had turned up to interview him. He was speaking from the 11th-floor of Donetsk's regional administration building, now a sprawling camp of anti-government and anti-western protest.

Pushilin describes himself as the "people's governor". He appeared to be reading from a carefully-drafted script. Several media advisers sat nearby. He told Russian television that Kiev was denying the local population access to insulin and withholding desperately needed medical supplies. He asked ordinary Russians to donate money to a numbered account with Russia's Sberbank to help the cause.

A local businessman, Pushilin and other deputies from the "Donetsk People's Republic" are entirely self-appointed. Their key demand is a referendum on federalisation by 11 May, two weeks before presidential elections. It is unclear what questions might be included.

Their goal is to create an autonomous eastern republic separate from Kiev. After that most want the new republic to join the Russian Federation, in imitation of Crimea annexed by Moscow last month. Kiev says Pushilin and other separatist leaders are under the control of Russia's spy agencies.

Visiting Donetsk on Friday, Ukraine's former prime minister Yulia Tymoshenko denounced Russian interference and said that Russia's special forces had been highly active across the east of the country. She said she was in Donetsk to negotiate with pro-Russian protesters, conceding that Ukrainian and Russian speakers now had to make "compromises" if a solution to the crisis was to be found. She said this compromise could be achieved if Russia withdrew its agents from eastern Ukraine but warned of violence if it did not.

Tymoshenko – whose pro-western party dominates the new government – said that she was creating a "resistance movement" militia to fight for Ukraine's territorial integrity. This would be an armed force made up of volunteers with military experience, she said: "We will do everything to restore harmony and peace in our country and to stop aggression. But if it doesn't happen we are ready to defend ourselves … with weapons in hand."

Tymoshenko ruled out holding a regional referendum, saying that it didn't match constitutional requirements, and adding that Kiev "can't recognise it". "We don't want anyone to demand that Ukrainians vote in a referendum under the barrels of Russian weapons," she said.