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

Ukraine has been in turmoil for months, since Mr. Yanukovych shocked much of the country by refusing to sign a trade deal with Europe and instead accepted a $15 billion loan package from Moscow. 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 on 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. A statement on the president’s website said Mr. 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 darkly 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.