President Yeltsin yesterday developed flu accompanied by severe bronchitis and was briefly taken to hospital for treatment, the Kremlin said.
Meanwhile Russian forces quickly encircled one of the last Chechen rebel strongholds as Moscow boosted war spending and dangled the prospect of a partial amnesty before the hardpressed Islamist rebels.
A senior aide to the President said doctors believed Mr Yeltsin's latest health problems could last for up to two weeks.
"The doctors say from one to two weeks, not taking into account the President's irrepressible character," the first deputy head of the Kremlin administration, Mr Igor Shabdurasulov, told NTV television.
Mr Yeltsin was meeting in the Kremlin with senior ministers ahead of a scheduled signing of a union treaty with Belarus today when suddenly "the President began feeling unwell", a spokesman, Mr Dmitry Yakushkin, told Russian news agencies.
Mr Yeltsin (68) was briefly taken to the Central Clinical Hospital, where he was diagnosed with "severe bronchitis" by his doctors, Mr Yakushkin said.
He was later moved to his official Gorky 9 suburban residence where he was undergoing treatment. Moscow this week was gripped by bitterly cold weather. Temperatures plunged to -20 degrees centigrade yesterday.
The President was last shown on television on Saturday, the day after he came back from a European security summit in Istanbul.
AFP reports from Grozny and Achkhoi Martan:
Some 20 Chechens were killed in a day of intense air and artillery bombardments by Russian forces, a Chechen military spokesman said yesterday. Russian air raids concentrated on Grozny, while heavy artillery shelling was reported around the key rebel stronghold of Urus-Martan.
Regions around the villages of Vedeno, Shatoi and Itum also came under attack.
The Chechen official said rebels were reinforcing their positions in Urus-Martan, denying reports that separatist forces were fleeing the town for safety in the mountains to the south.
Poor weather limited Russia's air force to only 15 helicopter attacks on rebel positions in Grozny, the capital of Chechnya, and Urus-Martan, the site of some of the most serious fighting this week.
But Russian forces made fast work of encircling Urus-Martan from the north and east, federal soldiers said.
The village 20 km south-west of Grozny has turned into a focal point of fighting this week as it remains the only open gateway to the rebel capital, which has been all but surrounded.
An elderly man who fled the village said that one field commander inside had said that he was hoping to lure Russian forces inside Urus-Martan, where the rebels had reportedly set up a fortified resistance.
The Russian Prime Minister, Mr Vladimir Putin, said on Wednesday that an amnesty would be offered to "all those carrying arms and are not on our side, but whose hands are not stained with the blood of Russian citizens."
Mr Putin for the first time has revealed that an extra three billion roubles ($115 million) has been earmarked for the campaign. He said the money came from extra revenues not foreseen in this year's budget.
But currency analysts suggest that Russia's Central Bank is spending up to $100 million daily to prop up the rouble this week - an unusually large sum that points to large-scale money printing by the government.
In Washington, the US Secretary of State, Ms Madeleine Albright, again condemned the offensive in Chechnya but said that financial assistance remained a different issue.
"I think it is very important to keep things separated here," she said, noting that economic stability in Russia was in the interests of the United States.