Russian troops were engaged in fierce combat today against hundreds of Chechen Islamic guerrillas who have seized control of six villages and a town in western Dagestan in the most intense fighting in southern Russia since the end of the war in neighboring Chechnya three years ago.

The Russian military threw tanks, artillery and air power into the conflict after an estimated 2,000 guerrillas entered Dagestan from Chechnya on Sunday in their second and largest cross-border thrust since early last month. Only days after claiming it had chased the Chechens back across the border, the military found itself drawn back into another guerrilla conflict in the region that will not be easy to suppress.

The rebels expanded their hold inside Dagestan today, according to reports from the region, as concern deepened in Moscow. Alexander Shokhin, a member of parliament, said a "full-scale undeclared war is going on" in the region, and President Boris Yeltsin called a meeting of his security council for Tuesday. Yeltsin's spokesman, Dmitri Yakushkin, said tonight that Yeltsin sees the Dagestan situation as a "serious threat to Russia's integrity."

Russian Su-24 light bombers and Su-25 attack planes flew 40 sorties over the region Sunday, in one case bombing the village of Zamai-Yurt on the Chechen side of the border, killing about two dozen people. The Interfax news agency reported that 32 houses were destroyed in the bombing and 100 more damaged. That attack came just hours after a car bomb claimed 56 lives at a five-story housing complex for Russian officers and their families in the Dagestani town of Buinaksk.

The guerrilla incursion was launched in an area where a major roadway crosses from Chechnya into Dagestan, and some of the fighting has spread to the outskirts of Khasavyurt, the second-largest town in the region, where a Russian-Chechen cease-fire was signed in 1996.

Military officials said the Chechens have occupied the villages of Chapayevo, Sushiya, Akhar, Novoluki, Tukhchar, Tamiyakh and the town of Novolakskoye. The Russian military said 14 soldiers were killed in a heavy firefight in Novolakskoye, when a column of troops came to the rescue of a number of local policemen who had been trapped in their headquarters by the Chechens.

Deputy Interior Minister Pyotr Latishev acknowledged in a television interview that the Chechens faced little resistance coming across the border. "There is no border line between Dagestan and Chechnya, no front line or system of border control," he said. "We have some posts along the border where there are roads. . . . The border is hundreds of kilometers long, and it is impossible to have guards every 100 meters."

From 1994 to 1996, Chechnya fought a bloody guerrilla war for independence from Moscow in which tens of thousands of people were killed. In the subsequent cease-fire, the question of Chechnya's political status was put off for five years, but some guerrillas remained armed and vowed to continue their struggle. They have been led in the recent fighting by Chechen militia commander Shamil Basayev, who led a bloody raid on the southern Russian city of Buddennovsk in 1995 in which more than 100 people were killed.

Their target now is Dagestan, where Basayev has said his goal is to create an Islamic republic. The region, a poor, mountainous territory about the size of Austria, is squeezed between the Caspian Sea on the east and Chechnya on the west. It is a patchwork of nationalities and has long been considered a laboratory for students of languages because of its diversity. The fighting has aroused passions among some Dagestani ethnic groups and clans who are arming themselves -- with the approval of the Russian military -- to fight the Chechens. There are also Chechen communities within Dagestan.

Vyacheslav Mikhailov, the Russian minister for federation affairs and nationalities, told the Tass news agency today that the Chechen guerrillas aim "to explode Dagestan at any cost."

The military response to the incursion drew criticism from some Russian legislators. "As prevention is not part of our defense and law-enforcement agencies' responsibilities, we missed the rebels' invasion into Dagestan altogether," said Gennady Seleznov, speaker of the lower house of parliament.