Vladimir Putin, Russia's acting prime minister, threatened today to send troops to fight once again in the breakaway southern region of Chechnya as Moscow dispatched additional military forces there.

The Russians charge that Chechnya is behind a recent incursion of up to 1,500 Islamic guerrillas into Dagestan, a neighboring Russian region. The aim of the incursion, according to Moscow, is to annex Dagestan to Chechnya, which declared itself an independent Islamic state in 1996 after defeating Russian troops in a two-year war. Moscow does not recognize Chechnya's independence.

Putin has taken a tough line on the Dagestan fighting, which is confined to the republic's mountainous western border with Chechnya. The conflict, Putin's first major challenge as prime minister, has erupted before his confirmation by parliament. The vote is scheduled for Monday.

"Chechnya is Russian territory, and wherever fighters are, strikes will be made against them," Putin told a group of Russian governors today. Russia sent several hundred reinforcements to Dagestan from the far north city of Murmansk.

In Chechnya, officials said Russian troops in five armored vehicles had crossed the frontier, but left within an hour.

The threat to send troops back into Chechnya is unsettling Moscow, because it would mean breaking a cease-fire and potentially plunging Russia back into a situation from which it retreated in humiliation. Some military analysts think the Russian military is in no better shape to fight the Chechens now than three years ago.

"Russia can blow out the lights in London and Washington and yet has failed time and again to contain relatively small bands of Chechen fighters on its territory," wrote Paul Felgenhauer in the Moscow Times.

The war in Dagestan already has been marked by contradictions. On one day, Russian officials claimed to have driven the rebels out of one of four villages under their control, only to announce the next day that the rebels held six other villages.

Russia says its troops have killed 150 rebels; the rebels say they have lost 10 men. Russia puts its own losses at 10, including an air force colonel killed in a mortar attack on an airport near the town of Botlikh.

Most of the Russian offensive, which began last Sunday, has been from the air. Russia is also arming local Daegestani villagers, on the grounds that they are no more eager for a Chechen takeover than is Moscow. "Of course, we won't give arms to bandits or religious extremists," said Mukhu Aliyev, the speaker of the Dagestani parliament, in response to Russian fears that the guns might be turned on government troops.

Today, televised images showed Russian helicopters and artillery raining rockets and shells on villages in the valley. Sixty miles inside Dagestan, the town of Buynaksk appeared to be under siege. Sandbagged bunkers were set up on street corners, and soldiers guarded government buildings. An army official said that off-duty troops are not permitted to wander around town, and always travel armed, and at least in pairs.

Viktor Kazantsev, the Russian military commander in the North Caucasus, dismissed the Chechens as bandits, but also said Russia must take precautions to defend Buynaksk from being overrun.