Russian forces fighting Muslim separatist guerrillas in the southern Dagestan region suffered their sharpest single setback today when eight soldiers were killed as they tried to storm a guerrilla-controlled hamlet.

The 12 days of conflict in the restive region have been rough-going for the Russians, and officials here apparently are beginning to prepare the public for a drawn-out conflict rather than the two-week operation promised as recently as Tuesday by Vladimir Putin, the new prime minister.

Police and defense officials now say that Moscow will have to send at least 10,000 troops--and perhaps 15,000--to drive the rebels out of Dagestan and back into neighboring Chechnya--a Russian region that has been virtually independent since Russian forces were forced out in 1996 by the same kind of guerrilla force now facing them in Dagestan. The fighting began early this month when the separatist guerrillas crossed into Dagestan and took control of several villages along the southern border of the quasi-autonomous region. About 4,000 Russian army and Interior Ministry troops are now deployed in Dagestan, facing an estimated 1,200 guerrillas, but as of today the Russians have a new commander.

Defense Minister Igor Sergeyev arrived today in Makhachkala, Dagestan's capital, and fired Vyacheslav Ovchinnikov, the chief of Interior Ministry forces in the region. Sergeyev went to Dagestan to inspect military operations there after President Boris Yeltsin asked him to take charge of suppressing the rebellion.

Ovchinnikov had been chief of operations around the strategically-situated town of Botlikh but had failed to organize his forces properly at the onset of the conflict, a Defense Ministry statement said. He has been replaced by Viktor Kazantsev, commander of Russian forces in the north Caucasus region.

Guerrilla commanders from Chechnya--whose political leaders deny any role in the cross-border incursion-- are apparently trying to expand their self-proclaimed Islamic republic to Dagestan and perhaps beyond. The battleground is a mountainous area of land along the Dagestan border, but for the Russians the stakes are greater. The Chechen-led offensive raises the nightmarish scenario for Moscow that its retreat from Chechnya could be repeated in Dagestan and then from the rest of the Caucasus region.

So far, the Russian military has relied largely on warplanes and helicopter gunships to counter the guerrillas. But clearing occupied villages will likely require infantry action, and today's toll exemplifies the risks of ground combat. Overall, about 25 Russians have been killed since the conflict began.

Today's fatalities occurred near Tando, a village overlooking Botlikh, according to Dagestani Interior Minister Adilgerei Magomedtagirov. While failing at Tando, the Russians said they took control of a mountain pass on the Chechen frontier, making it difficult for the guerrillas to reinforce and supply themselves.

A Chechen spokesman said the guerrillas crippled three Russian tanks and seven armored vehicles as they turned back the Tando attack.

CAPTION: Officials say Russia will have to send at least 10,000 troops to Dagestan--such as these preparing to raid a village there earlier this week--to drive Islamic rebels out of the region and back to Chechnya.