Prime Minister Vladimir Putin said today that Russian troops have now occupied the northernmost third of Chechnya, but he declared that the military operation in the separatist southern region is not yet over.

Speaking to a group of prominent legislators and former prime ministers, Putin indicated that the goal of the advance into Chechnya is significantly more ambitious than the one announced two weeks ago of creating a security zone around Chechen territory to prevent Islamic guerrillas based there from infiltrating neighboring Russian regions.

Now, Putin said, Russian troops are trying to establish a security zone within Chechnya from which to press a full-scale campaign against the guerrillas. "This is just one stage in this operation," Putin said. "But the ultimate aim is to fully destroy terrorists and their bases throughout Chechen territory."

While the precise nature of Russian troop movements has been kept secret, Chechen and Russian reports from the region agree that Russian troops have advanced to or near the Terek River, a key regional landmark that runs through Chechnya from west to east about 25 miles north of Grozny, the capital.

The northern part of Chechnya, which is relatively flat compared with the mountainous south, has traditionally been less hostile to Russian control. According to Putin, Russian troops suffered only light casualties as they advanced toward the Terek, with four soldiers killed and 22 wounded, compared with the hundreds who died fighting Chechen Islamic guerrillas in August and September after the Chechens seized several towns and villages in neighboring Dagestan. But Putin's figures could not be verified, and the Russian military has consistently underestimated casualties in the past.

Kremlin officials have repeatedly pledged to avoid a costly ground offensive--like the disastrous advance of Russian troops against Chechen separatist forces over some of the same ground in late 1994--and they have been bombarding Chechen targets and civilian villages for two weeks. Today, officials here reported the loss of two fighter-bombers, an Su-24 and an Su-25--the two aircraft types that have carried out the bulk of Russia air attacks on Chechen targets. Chechen defenders claimed they downed one of the planes with a Stinger antiaircraft missile.

In the ground advance, television reports said Russian units are moving cautiously through villages, often with their commanders asking local elders to expel Chechen militiamen before Russian troops move in. The reports said the commanders hope thereby to avoid the heavy casualties that characterized the Russian military campaign in Chechnya of 1994-96. In that conflict, Chechen guerrillas inflicted heavy losses on the slow-moving, ill-equipped Russians with ambushes and surprise attacks, demoralizing them and ultimately driving them out of the territory.

Putin's announcement today appeared to be an effort to reinforce political support for the expanding ground operation in Chechnya. So far, the military effort has been popular here in the aftermath of a series of bomb attacks on apartment houses in Moscow and other Russian cities in which 300 civilians died and which Russian authorities have blamed on Chechen "terrorists."

Putin won pledges of support from politicians today, but doubts also were voiced after the meeting. Grigory Yavlinsky, the leader of the centrist Yabloko bloc in parliament--who was an opponent of the 1994-96 war but enthusiastically backed the latest operation at the outset--said today that "we are very much concerned about the way events are going to develop" in the region.

"We need less haste and less it's-in-the-bag sentiment," he said, "so that what happened in Chechnya several years ago doesn't happen again."

Sergei Stepashin, a former prime minister who was closely involved in the opening phase of the last war, also sounded a warning. "One cannot start a war without a plan," he said. "What is going to be the end of the war? The terrorists and bandits must be destroyed, that's clear. But . . . who is going to negotiate with us? We must look for partners."