Ukrainian President Viktor Yanukovich has gone on sick leave with an acute respiratory ailment, leaving a political vacuum in a country destabilised by anti-government protests.

Yanukovich's prime minister, Mykola Azarov, resigned on Tuesday after a sharp escalation of two months of street unrest prompted by Yanukovich's rejection of a European Union deal in favour of closer ties and a bailout deal with Russia.

The president, increasingly isolated at home and under pressure from Moscow not to tilt policy back towards the West, has yet to appoint a successor. Serhiy Arbuzov, who was Azarov's first deputy and is a close family friend of Yanukovich, has stepped in as interim prime minister.

"The president of Ukraine has been officially registered as sick with an acute respiratory ailment and a high temperature," at statement on the presidential website said on Thursday.

The bare announcement, which gave no indication of when he might be back at his desk, left open the possibility that he could still work on documents even though he was on sick leave.

"Today is the first day of the illness. He has a high temperature. We are not doctors, but it is clear that a high temperature does not go down in a single day," a presidential spokesman said by telephone.

"The doctors will do all they can so that he can recover quickly."

Many anti-government protesters on the streets of Kiev greeted the news with derision.

"Yanukovich has never worked this hard before, so he got exhausted and needed a break," Sergei, an activist from Ivano-Frankivsk Oblast, who did not give his surname, told Al Jazeera.

Amnesty rejected

Earlier, the anti-government protesters rejected an amnesty bill that would free activists and said Russian President Vladimir Putin's decision to hold off on releasing his country's next bailout instalment will not impact their stance that Yanukovich should resign.

The offer was quickly greeted with contempt by the opposition and protesters said the moves were insufficient.

"We will seize new buildings if the authorities don't really change the situation in the country," said 30-year-old Artem Sharai, demonstrating on Kiev's central Independence Square.

Protesters are demanding Yanukovich's resignation, early elections and the firing of authorities responsible for violent police dispersals of demonstrators.

"Instead of lowering the temperature in society this is going to raise it" said Vitali Klitschko, leader of the Ukrainian Democratic Alliance for Reform party (UDAR).

In a move likely to further complicate Ukraine's situation, Vladimir Putin said on Wednesday that Moscow would wait until a new Ukrainian government is formed before it considers releasing a crucial $15bn bailout package to Kiev in full.

Analysts said the Kremlin may renege on the deal if Ukrainians elect an anti-Russian government and warned that the European Union might have to bailout Ukraine.

"Neither Russia or Europe should influence what is happening in Ukraine, because it is the business of Ukrainian people how to live on," anti-government activist Nikolai told Al Jazeera, refusing to give his surname.

Dozens of activists have been arrested since clashes broke out on January 19 and protesters stormed regional administration buildings in Kiev and the provinces.

Three activists were shot dead last week when clashes broke out between police and protesters in Kiev, but tensions have calmed somewhat since negotiations began between the government and opposition.

Additional reporting by Tamila Varshalomidze in Kiev.