Russian military commanders announced today they have completed the first phase of operations against Chechen fighters and suggested that the second stage will begin this weekend with deeper advances into Chechnya.

After first saying that their aim was to seal Chechnya at its borders, the Russian military went further, seizing the northern third of the rebellious southern region. Commanders then said they did not intend to go south of the Terek River, which bisects Chechnya, but today there were strong suggestions they would.

In a battle for one town, Goragorsky, the Russians have already moved south of the river and today said they had captured the hilly outpost that overlooks a main route to the Chechen capital, Grozny, about 1,000 miles south of Moscow. In the last Chechen war, which ended in 1996, Russian soldiers suffered heavy casualties storming Grozny, and this time they will avoid an assault on the capital.

Col. Gen. Viktor Kazantsev, commander of Russian troops in the region, said the second phase will begin Saturday. He did not provide details, saying he wanted the Chechen rebels to "expect us anywhere at any time and to suffer losses all the time."

Lt. Gen. Igor Zubov told reporters here that the "security zone" created in northern Chechnya and on the region's eastern and western borders--to bar Chechen guerrillas from incursions into adjacent Russian territories--was only part of the Russian plan. The zone "cannot fully resolve the matter," he said. "If it becomes necessary . . . the troops will go to the place where it is necessary to resolve the main task, to put an end to attacks against the neighboring subjects of the Russian Federation."

Asked if Russia is ready for a guerrilla war in the mountains, he said, "We are preparing, including for this." He suggested that the Russian troops would have to make advances on the ground and could not fight the entire battle from the air.

The Chechens claim they have downed three Russian planes since the conflict began last month, but Russian officials have given conflicting accounts. The Russians have charged that the Chechens are using Stinger shoulder-fired antiaircraft missiles acquired from the Taliban militia in Afghanistan.

The Russians said they have lost 47 men since the Chechen advance began; the Chechens claim to have killed hundreds.

Partly to compensate for the difficulty of airstrikes, the Russians said they launched tactical missiles this week at Chechen villages southwest of Grozny. About 156,000 Chechen refugees have now fled into neighboring Ingushetia in the west.

Russia has vowed to set up a puppet government in the lands it controls in northern Chechnya, and today President Boris Yeltsin, recuperating from the flu in a country residence, appointed Nikolai Koshman as special envoy. Koshman served as prime minister in a similar government Russia attempted to set up during the 1994-96 war, which ended with a cease-fire that deferred the question of Chechnya's status for five years.

The Russians mounted the most recent offensive after a series of terrorist bombings, which Russia blames on Chechen rebels, killed nearly 300 people in Moscow and other cities. The Russian operation also comes in the wake of border incursions by Chechen fighters into neighboring Dagestan that started in August.