Ukrainian President Viktor Yanukovych, who is facing calls for his resignation amid a severe political crisis, has gone on sick leave, his office said Thursday.

"Ukraine's president is on sick leave in connection with an acute respiratory disease accompanied by high fever," the office said in a statement, citing the deputy head of the State Directorate for Medical Affairs, Olexander Orda.

Yanukovych attended a caucus meeting of his Party of Regions on Wednesday evening and urged lawmakers to pass a conditional amnesty for demonstrators detained during protests over the past two months.

The amnesty bill, approved shortly before midnight Wednesday by the Party of Regions and the allied Communist Party, will only take effect after opposition activists abandon all the administrative buildings they have occupied in Kiev since the mass protests began in November.

The demonstrators have 15 days after the amnesty law takes effect to comply with those terms. The opposition parties, which were demanding an unconditional amnesty and did not participate in the voting, have vowed not to do so.

The anti-government protests, which had been mostly peaceful until last week, began in Ukraine in late November when the government backed away from plans to ink a free-trade deal with the European Union.

Yanukovych later secured a $15 billion aid package from Russia and negotiated a deal to enable Ukraine, a former Soviet republic, to purchase natural gas from its neighbor at a steep discount.

Amid the growing pro-Europe demonstrations in downtown Kiev, Ukraine's Parliament, controlled by Yanukovych's allies, passed a package of laws on Jan. 16 restricting the right to gather in public, freedom of speech and other basic rights.

Anger over the anti-protest legislation spilled over into clashes three days later in Kiev pitting demonstrators demanding Yanukovych's resignation against riot police. At least three people were killed and hundreds of others were wounded in the street battles.

Authorities and opposition leaders agreed to a truce on Jan. 23.

During subsequent talks, the opposition has secured most of its objectives - including the resignation of Prime Minister Mykola Azarov and the repeal of the anti-protest laws - though the government has so far rejected its call for early presidential and parliamentary elections.

On Thursday, thousands of demonstrators were continuing to occupy Kiev's Independence Square to press their demands. EFE