Russia replaced two of its three senior field generals in Chechnya today and announced it was indefinitely suspending airstrikes and artillery bombardment of Grozny, the capital of the breakaway southern region.

The unexpected announcements came as a Russian offensive stalled on the outskirts of Grozny in the face of fierce resistance from separatist rebels, thick fog and other bad weather. The military said ground fighting around Grozny will continue.

The reason behind the decision to suspend the bombardment of Grozny was not entirely clear. The Russians said it was out of concern that chlorine gas released by the rebels was endangering civilians, who have been trapped in the city for weeks by Russian shells and rockets. The Russians said the rebels blew up chlorine containers when Russian troops threatened to enter the city, but they said the gas was released over the past two months and did not explain why concern was not aroused until now.

At the same time, two senior generals who have commanded the eastern and western fronts of the military offensive in Chechnya were replaced. Gen. Gennady Troshev, the eastern commander, and Gen. Vladimir Shamanov, the western chief, were removed from the combat zone, Troshev announced.

The Defense Ministry said the decision was "not a consequence of any professional errors" by the two field commanders, who have helped lead the three-month-old ground offensive by 100,000 Russian soldiers against separatist Chechen rebels. After seizing control of the northern two-thirds of Chechnya, the military on Dec. 25 launched an all-out assault to drive the guerrillas from Grozny. But the assault has stalled, arousing some Russian concern that the operation could turn into a protracted guerrilla conflict similar to the two-year war that ended in 1996 with Chechnya's virtual independence.

The announcement came just a day after Troshev and Shamanov rejected a call for a three-day cease-fire by Chechen President Aslan Maskhadov.

There have been unconfirmed reports for more than two weeks that Shamanov would be replaced because of a battle in early December in the Chechen town of Alkhan-Yurt, where witnesses said soldiers under his command went on a rampage, looting property and killing civilians who were in their way. At least 17 and perhaps up to 40 civilians were killed.

Then-President Boris Yeltsin later awarded Shamanov and Troshev the Hero of Russia medal.

Troshev said he and Shamanov were leaving to give other generals combat experience. "The moment has arrived when someone else should have a go," he said, adding that both generals were being replaced by their deputies.

The Defense Ministry said the general staff had made the rotation so "other commanders can gain combat experience," the Interfax news agency said. Troshev will be replaced by Gen. Sergei Makarov and Shamanov by Gen. Alexei Verbitsky. Troshev will return to his former duties as deputy commander of the North Caucasus Military District, which includes Chechnya, while Shamanov will return to his post as commander of the 58th Army in the same district.

Shamanov has been a particularly outspoken general. When reports surfaced in October that the Kremlin might be considering peace talks with the Chechens, Shamanov openly warned that he would resign, and that other officers would follow him. His remarks reportedly infuriated the Kremlin.

The announcement of the bombing pause came at a news conference by Troshev at the regional military headquarters in Mozdok. "We have been forced to suspend military operations all around Grozny for one single reason," he said. "There are still civilians there who fighters kept behind deliberately to use as human shields." He added that "the fighters, as you know, in the last two months have twice used chemical weapons--well, maybe that is overstating it. But at least they have blown up the containers they prepared for the moment when troops enter the city, which is basically what happened."

Previously, the Russians have said the chlorine was blown away by winds. Troshev said that while Russian soldiers have gas masks, civilians do not, and this discrepancy led to the decision to suspend the bombing.

"The area of the city of Grozny was today officially declared a risk zone, a zone of ecological danger," Troshev said. "First and foremost this will affect civilians, children. So that's why the group command decided to suspend operations. Suspend does not mean discontinue military operations. Military operations are being conducted around the clock."

Troshev did not say how long the suspension would last. He also did not address how much the military's near-constant bombardment of Grozny puts the city's civilians at risk. Tens of thousands of civilians are believed trapped in cold, dark basements. They have been reluctant to leave because of the dangers of Russia's onslaught, and it is not clear that the bombing pause will encourage them to leave now.

Russian airstrikes on the Chechen fighters in other areas continued today, including in the southern mountains. The military said Russian warplanes made more than 40 combat flights in the last 24-hour period.