Ukraine's embattled president, Viktor Yanukovych, has accused the opposition of escalating the situation in the country and insisted that his government is doing all it can to solve the crisis.

Yanukovych's defiant statement – released on the presidential website – followed an announcement that he is taking sick leave due to an acute respiratory illness and high fever. There was no indication of how long he might be on leave or whether he would be able to do any work. The chief medical officer said the president was suffering from a "severe cold with a high temperature".

The opposition UDAR party led by Vitali Klitschko responded with a statement saying that the sick leave "gives the impression he is trying to stay aloof from the political crisis rocking the country".

Yanukovych accused the opposition of "continuing to whip up the situation, calling on people to stand in the cold for the sake of the political ambitions of a few leaders". He is under pressure after two months of major protests seeking his resignation, early elections and other demands.

Anti-government protesters in Kiev's Independence Square. Photograph: Rob Stothard/Getty Images

On Tuesday Yanukovych's hardline prime minister, Mykola Azarov, quit in an apparent concession to the opposition. The president has yet to appoint a successor. Azarov's deputy, Serhiy Arbuzov, has taken over as interim PM.

Yanukovych's illness is another unpredictable element in Ukraine's already combustible political drama. His predecessor, Viktor Yushchenko, fell dramatically ill in 2004 after apparently being poisoned with dioxin, while standing against Russian-backed Yanukovych in the presidential election.

On Wednesday, meanwhile, Ukraine's parliament passed a law that would grant an amnesty to arrested protesters but – to the opposition's fury – depended on the demonstrators vacating all occupied government buildings. Yanukovych turned up in parliament late on Wednesday evening for the vote, looking pale. He is said to have told supporters that he had been receiving treatment in hospital.

After 12 hours of negotiations the amnesty law was passed amid applause from the ruling party and angry shouts of "shame!" from the opposition.

Over 50 members of Yanukovych's Regions party were willing to approve a milder version of the draft law that would have allowed the unconditional release of protesters arrested during clashes. But the president insisted that the amnesty could only take place if occupied administrative offices were vacated in the 15 days after the new law comes into force. The law passed by 232 votes.

Protesters are holding three administrative buildings in Kiev, including the building housing the city administration. With temperatures in Kiev falling to as low as -20C at night, the buildings are vital for the opposition's ability to continue street protests and maintain pressure on the government.

Vitali Klitschko. Photograph: Zurab Kurtsikidze/EPA

Yuri Miroshnychenko, Yanukovych's representative in parliament, said the protesters would now have to leave the buildings. But he insisted the opposition headquarters in Trade Union House, as well as Independence Square and Khreschatyk Street, where the protest camp is located, would not be touched.

The offer was quickly greeted with contempt by opposition protesters. Klitschko said the law "will only increase the temperature in society", while Andriy Parubiy of the Batkivshchyna party called the demands unacceptable. "No one will comply with them," he said.

Ukraine's political uprising began more than two months ago as a pro-European movement after Yanukovych rejected an integration pact with the European Union. Instead he accepted a $15bn (£9bn) bailout from Russia. The Kremlin has now said it may fail to pay the loan in full if the government in Kiev changes.

The protests have since spread across much of the country, and have turned into a highly personal campaign to topple Yanukovych. Radical groups clashed with the police last week, with at least three civilians killed. Several dozen opposition activists have disappeared.

Amid an escalation that turned Hrushevskoho Street in the centre of Kiev into a kind of battlefield, officials even mooted imposing a state of emergency in the country.

Parliament has voted to repeal a series of draconian laws hurriedly passed on 16 January, which outlawed freedom of assembly and ushered in – according to critics – a form of dictatorship. Yanukovych has yet to ratify the vote, which means that the punitive laws still stand.

Read more

Seumas Milne: fascists, oligarchs and western expansion at heart of crisis

Andrey Kurkov: my dream home is now a war zone. But I'm not quitting