President Boris Yeltsin suddenly fell ill today on the eve of a ceremony to sign a treaty related to the unification of Russia and Belarus, which was abruptly postponed.

Yeltsin, who has been in and out of hospitals more than a dozen times in the past three years, was taken to the Central Clinical Hospital with what his press secretary described as "a viral infection and acute bronchitis." He was reported to be resting tonight at Gorky-9, his residence outside Moscow.

Yeltsin, 68, made a vigorous speech at the Organization for Security and Cooperation in Europe summit in Istanbul last week, but has not been seen in public since then. He underwent coronary artery bypass surgery in 1996 and has suffered from respiratory illnesses every winter since then, as well as an ulcer and other complications.

The announcement late this afternoon caught many by surprise because the Russian president had appeared in the Kremlin earlier in the day to preside over a meeting of the Security Council, a high-level policy body that includes defense and security officials. The war in Chechnya and the upcoming signing ceremony for the agreement with Belarus were on the agenda.

As always, reports about Yeltsin's health were far from precise. While press secretary Dmitri Yakushkin said the president was taken to the hospital, Prime Minister Vladimir Putin said Yeltsin "has just a sore throat." A similar explanation was given in 1996 when Yeltsin canceled some campaign trips and suffered a heart attack.

Putin said "there is nothing serious," after speaking with Yeltsin by phone. He said Yeltsin was "hoarse." Under the Russian constitution, Putin would become acting president for three months should Yeltsin die or become incapacitated.

"We are not doctors," said Yeltsin's deputy chief of staff, Igor Shabdurasulov. "We cannot judge how serious the sickness is. . . . The doctors expect it will take a week or two, but knowing the president's energetic nature, you know, I think he feels vexed that everything happened this way."

The illness, coming on the eve of the treaty signing, led to some speculation that perhaps Yeltsin was trying to dodge the event, but aides insisted he was not attempting to scuttle the plan. Aides said the signing ceremony would be rescheduled for December.

The treaty is the latest in a string of efforts--so far largely symbolic--to bring Russia and Belarus closer to unification. Repeated vows to merge economic ties have not been implemented, but the idea is popular among some nationalists and Communists.

One big enthusiast is the president of Belarus, Alexander Lukashenko, whose increasingly authoritarian rule has repelled many Russian liberals. They fear that Lukashenko is seeking unification with Russia to advance his personal ambition of becoming a leading political figure here. Russian liberals also say Belarus has taken none of Russia's difficult steps toward a market economy.