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The 63-year-old president appears increasingly isolated in a crisis born of a tug-of-war between the West and Ukraine's former Soviet overlord Russia. A former president said this week the violence between demonstrators and police had brought the country to the brink of civil war.
Shortly after his office announced he had developed a high temperature and acute respiratory ailment, Yanukovich defended his record in handling the crisis and accused the opposition, which is demanding his resignation, of provoking the unrest.
"We have fulfilled all the obligations which the authorities took on themselves," a presidential statement said, referring to a bill passed late on Wednesday granting a conditional amnesty for activists who had been detained.
"However, the opposition continues to whip up the situation, calling on people to stand in the cold for the sake of the political ambitions of a few leaders."
The amnesty offered freedom from prosecution to peaceful protesters, but only on condition that activists left official buildings they have occupied - something they have rejected.
Several members of Yanukovich's own party voted against the bill, even after he visited parliament himself to rally support, and some of his powerful industrialist backers are showing signs of impatience with the two-month-old crisis.
Prime Minister Mykola Azarov resigned on Tuesday after a sharp escalation of the street unrest, which began in November when Yanukovich rejected a European Union deal in favour of closer ties and a bailout deal with Russia.
The president, under pressure from Moscow not to tilt policy back towards the West, has yet to appoint a successor. Serhiy Arbuzov, Azarov's first deputy and 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," a statement on the presidential website said.
A subsequent statement gave fullsome tribute to a police officer who was found dead early on Thursday, apparently from a heart attack while on duty - an indication of how important Yanukovich regards keeping the security forces on his side.
The bare announcement on his health gave no sign of when he might be back at his desk or able to appoint a new government, which Moscow says must be in place before it goes ahead with a planned purchase of $2 billion of Ukrainian government bonds.
"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."
Some opposition figures said they suspected Yanukovich might be giving himself a breathing space after being forced into concessions to try to calm the unrest on the streets.
"This smacks of a 'diplomatic illness'," Rostislav Pavlenko, a member of boxer-turned-politician Vitaly Klitschko's Udar (Punch) party, told Reuters. "It allows Yanukovich not to sign laws, not to meet the opposition, absent himself from decisions to solve the political crisis."
A close ally of Yanukovich, who was last seen in parliament on Wednesday night, rejected that interpretation.
The president had hurried to the legislature to herd supporters into voting for the amnesty bill. Mykhailo Chechetov, from Yanukovich's Party of Regions, said the president had told supporters there he had come to the session directly from hospital. "He looked ill," Chechetov said.
Photographs released by the presidential press service of Yanukovich holding talks with a European Union delegation earlier in the day revealed no obvious signs of illness.
In a statement the three main opposition leaders, including Klitschko, accused Yanukovich of ignoring violations of voting procedure in the Wednesday night vote.
"Viktor Yanukovich bears responsibility for the violations of constitutional norms ... (he) personally went to parliament and by blackmail and intimidation forced his faction, which is balanced on the edge of a split, to go back in and push through a law even when there were not enough votes for it," they said.
Thirty-year-old Ruslan Andriyko, one of the hundreds of protesters occupying Kiev's City Hall, said it would not work.
"We will clear this building only if we get the resignation of Yanukovich, which is the main aim of our revolution, and the approval of the people on the 'Maidan' (Kiev's Independence Square)," he said.
The president has not had a history of ill health. He has full control over the government and still has solid backing in parliament but there are signs of discontent in his Party of Regions over the continuing crisis on the streets.
He replaced his long-standing head of administration in mid-January and has since sacked his press secretary.
Former Polish President Aleksander Kwasniewski said he believed the hasty visit to parliament was a sign Yanukovich, who he has met many times, was afraid of losing support.
"I think this urgent visit by the President to parliament shows he is afraid that the majority is no longer on his side," Kwasniewski said on Polish radio.
Ukraine's richest entrepreneurs, whose support Yanukovich has had and needs now, are now taking a more neutral line.
Chemical and gas billionaire Dmitry Firtash called on all sides in the conflict to find a compromise by negotiations that would yield "real" results, according to a statement from him on Thursday. Ukraine's richest man, steel magnate Rinat Akhmetov, made a similar appeal earlier this week.
Yanukovich's most urgent task now is to appoint a successor to Azarov, who served him loyally for four years, while the opposition is anxious that he also signs into force a repeal of anti-protest legislation.
Ukraine badly needs a new government. Russian President Vladimir Putin said on Wednesday Moscow would wait until one was formed before fully implementing the $15 billion bailout deal.
The cost of insuring Ukraine's debt against default rose to a new one month high on Thursday, and Ukraine's central bank intervened for a fourth successive day, offering dollars on the inter-bank market to prevent a serious slide in the national currency, the hryvnia, from its peg at around 8 to the dollar.
The statistics agency said the economy, dominated by steel exports, had ground to a halt in 2013. Analysts expect output to fall this year.
Six people have been killed and hundreds have been injured in street battles between anti-government demonstrators and police which escalated sharply after the authorities toughened their response. The police officer who died on the street on Wednesday night took the death toll to seven.