Buildings in Donetsk and Luhansk remain occupied despite an offer of amnesty for those who leave peacefully

Rumours of imminent military assaults on the government buildings seized by pro-Russian protestors kept tensions simmering in eastern Ukraine on Thursday, despite an offer of amnesty for those who leave the buildings peacefully.

After pro-Russian protesters demanding referenda on greater autonomy from Kiev stormed government buildings in the eastern regional capitals of Donetsk, Kharkiv and Luhansk over the weekend, rumours of a military response by the Ukrainian authorities have run rampant. While negotiations have been held in recent days, reports on Thursday suggested government forces were poised for a possible attack. A masked guard told those entering the occupied Donetsk building on Thursday afternoon: "You've been warned – you may not come out!"

Although police managed to quickly evict protestors from the Kharkiv administration building and a security service building in Donetsk, the Donetsk administration building and the security service headquarters in Luhansk remain in protesters' hands. But interior minister Arsen Avakov said on Wednesday morning that "a solution to this crisis will be found within 48 hours", noting that an "anti-terrorist operation" was ready to spring into action at any moment. Avakov's suggested deadline will run out on Friday morning.

A Donetsk news publication tweeted a photo on Thursday of a long line of transport trucks and artillery it said was a Ukrainian military deployment on the outskirts of the city. Television news reports later said locals armed with clubs had blocked an artillery brigade moving toward Donetsk and forced it to turn around.

The situation was also fraught in Luhansk, where combat veterans calling themselves the Army of the Southeast have kept a tight hold on the security service building and reportedly seized a cache of machine guns. Protesters outside the building continued to stockpile Molotov cocktails and laid down spike strips to puncture tyres on nearby roads, Ukrainian news agency UNIAN reported. On Thursday morning, police cleared 10 vehicles that had blocked off a military base in Luhansk late the night before, according to local media.

MP Andriy Senchenko, the acting deputy head of the presidential administration, said security council secretary Andriy Parubiy and security service head Valentyn Nalivaichenko were located in Luhansk and would decide on further actions there. This news would appear to bring old enemies face to face once again: Parubiy came to prominence as the head of the self-defence forces at the Euromaidan demonstrations in Kiev, while members of the Luhansk occupation told the Guardian on Wednesday that they had fought protestors in Kiev as officers of the now-disbanded elite Berkut riot police.

Ongoing negotiations have yet to produce results. Acting president Oleskandr Turchynov said on Thursday he had offered several compromises during a telephone conversation with the protesters holding the building in Luhansk, including possibly expanding the powers of local governing bodies. He added that he was ready to offer similar compromises to Donetsk protestors, who declared a "people's republic" in the region on Monday.

Donetsk leaders told journalists on Thursday negotiations had made no progress since meetings with local authorities earlier this week, which included the participation of local coalmining baron Rinat Akhmetov and the governor recently appointed by Kiev, billionaire Serhiy Taruta.

Protesters in the two coal-mining and metalworking regions seek a referendum on sovereignty, which they say would not necessarily mean becoming a part of Russia but would allow closer economic ties with Ukraine's traditional export partner. But they have also called on Russia to help them hold such a referendum. The Donetsk "people's republic" announced on Thursday it had formed a committee on international relations that would open a dialogue with Russia's Customs Union, a body Vladimir Putin has promoted as an alternative to the European Union.

In a play to defuse the situation, Turchynov also offered to issue a decree to amnesty those who laid down their weapons and left the occupied buildings voluntarily. But Senchenko later added a caveat, saying that "people who have committed grievous crimes" would not be amnestied.

Inside the occupied Donetsk administration building, the offer of amnesty was met with derision.

"We don't recognise this regime, how can we recognise their laws?" Denis Pushilin, head of the republic's self-declared temporary government, told journalists. He accused the regime of holding double standards, arguing that it had not yet disarmed nationalist militias who supported the ouster of former president Viktor Yanukovich.

A Cossack from the nearby city of Mariupol guarding the entrance downstairs called the amnesty offer "funny" and said protestors would "stand until the end".

"Amnesty is given only when a person committed a crime and has been sentenced. What am I guilty of? This nonexistent regime wants to judge me?" said the Cossack, who would identify himself only as "Mortal," a nickname he said he received fighting in the Russian army during the first Chechen war.

The unrest in Donetsk and Luhansk could threaten the 25 May presidential elections that Kiev authorities have said are to fully legitimise the new government. The protesters refuse to recognise the Kiev government or the planned elections, which don't include a pro-Russian candidate among the frontrunners, and promise to hold a referendum on the sovereignty of Donetsk and Luhansk by 11 May.

Billionaire MP Sergei Tigipko, who previously held high government positions and has recently curried favour with the pro-Russian protestors in the east, said in Odessa that "presidential elections might not happen" depending on "how the situation in southeast Ukraine develops". Pro-Kiev activists later pelted the former banking tycoon with eggs, calling him "Putin's whore".

But pro-European presidential candidate Petro Poroshenko, known as the "chocolate king," who currently leads in the polls, said on Thursday that any delay of the elections would be "treason" and would not happen no matter the circumstances.

A survey published on Thursday by the Donetsk-based Institute of Social Research and Political Analysis found mixed opinions on the region's future: 18.6% of Donetsk residents oppose changes to Ukraine's government structure, 31.6% support greater regional control over the economy and taxation, and 15.5% support the federalisation of the country. Almost 27% supported the unification of either Donetsk or all Ukraine with Russia.