KIEV, Ukraine — Critical negotiations between the embattled Ukrainian government and opposition leaders were thrown into disarray Thursday when President Viktor Yanukovych went on sick leave, complaining of a respiratory infection.

Ukraine has been in turmoil for months, since Yanukovych shocked much of the country by refusing to sign a trade deal with the European Union and instead accepted a $15 billion loan package from Russia.

But he has found himself caught between the competing demands of the protesters in the streets of Kiev and other Ukrainian cities and his allies in the Kremlin, who suspended the loan deal Wednesday after disbursing only $3 billion.

The nature and timing of the president’s illness raised immediate questions about his true motive for withdrawing from the political fray when negotiations with the opposition seemed to be gaining momentum.

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A statement on the president’s website said Yanukovych, 63, was taking time off because of “respiratory illness accompanied by a high temperature.” It offered no indication of how long he was expected to be absent.

Some opposition figures speculated that the president was removing himself from the scene in preparation for declaring a state of emergency, a last-ditch measure that the protesters have been warning against for weeks, saying it could ignite an all-out civil war.

“I remember from the Soviet Union it’s a bad sign; a bad sign because always if some Soviet Union leaders have to make an unpopular decision, they go to the hospital,” said Vitali Klitschko, the former boxing champion who leads the opposition party Ukrainian Democratic Alliance for Reform.

Vitali Portnikov, an opposition journalist, suggested that rather than a virus, Yanukovych was falling prey to internal political pressures, perhaps losing power to a hard-line faction in his government, a development that could presage a coup d’état.

“I don’t remember official statements of Viktor Yanukovych having a cold,” he wrote in a Facebook post. “But I remember well that on the 19th of August, 1991, the vice president of the USSR, Gennady Yanayev, announced a serious illness of Mikhail Sergeyevich Gorbachev.”

The next day, Gorbachev was arrested as part of a failed coup.

Yanukovych’s sick leave took effect before he could sign a bill repealing restrictions on freedom of speech and assembly that were enacted this month. The repeal was passed by Parliament on Tuesday with support from the pro-government Party of Regions, a significant concession to the opposition but one that means little unless the president signs it.

The rollback and other measures negotiated with the opposition this week had seemed to open the way for a possible ceding of some power by the government, potentially quieting the crisis atmosphere that has enveloped the capital for weeks.

The president, though, is facing pressure from Russia to take a harder line with protesters, rather than continue negotiations. The loans were suspended, the Kremlin said, until it became clear what sort of government would emerge from the current negotiations.