Russian planes dropped leaflets today over Grozny, the devastated capital of breakaway Chechnya, delivering a stark ultimatum to civilians: Leave by Saturday or face intensified air and artillery strikes.

Leaflets also warned Grozny's defenders to give up or die, and officials said new, heavier armaments will be used to batter the city. "Everyone who fails to leave . . . will be destroyed," the leaflets said.

Russian officials described the ultimatum as the start of a new phase in the combat against the separatist region. Rebels will be wiped out or expelled from urban areas and forced into the mountainous south, where they will be pounded by pursuing jets and artillery, they said.

The two ultimatums support the growing perception that Russian generals urgently want to retake the city from which they were expelled three years ago at the end of Chechnya's independence war. They appeared intent on creating an urban free-fire zone in which anything that moves will be considered a legitimate military target.

Up to 20,000 civilians remain in the city, the Russians said. Chechen officials put the number at 40,000.

President Clinton sharply criticized the Russian ultimatum to civilians, calling it "a threat to the lives of the old, the infirm, the injured people and other innocent civilians who simply cannot leave or are too scared to leave their homes."

"Russia will pay a heavy price for those actions, with each passing day sinking more deeply into a morass that will intensify extremism and diminish its own standing in the world," Clinton said at the White House.

Moscow has repeatedly said that the expressed aim of the conflict is to free Chechnya of "terrorists" and restore Russian rule. Prime Minister Vladimir Putin has elevated the war to a battle for the very existence of Russia. Over the weekend, he responded to intense Western criticism of the campaign's brutality by saying Moscow could not be expected to build friendly relations with foreign countries "at the price of breaking up our state."

There is political significance for Putin in the timing of the planned offensive. Russian parliamentary elections are scheduled for Dec. 19, and the results are regarded as a bellwether for next year's presidential vote. Putin has staked his presidential candidacy on the war's outcome, and so far his popularity has soared. He backs the Unity party in the parliamentary vote. Since his recent endorsement, polls show Unity gaining in popularity. A triumphant march into Grozny would seal Putin's popularity, and perhaps Unity's.

The Russian advance toward Grozny has been stalled in recent weeks because of Russian caution about casualties among its forces and stiff resistance by Chechen guerrillas in two towns that flank the city: Urus-Martan to the southwest and Argun just to the east, where fighting continues. Troops entered Argun today, three days after its announced capture.

"For the sake of avoiding victims within the peaceful population, we ask you to leave Grozny before Dec. 11, using all possible ways," said the leaflet addressed to Grozny residents.

The defenders were told simply to give up. "For those who have not yet lost their senses . . . you are surrounded. All roads to Grozny are blocked. You lost.

"Those staying in the city will be regarded as terrorists and bandits. They will be destroyed by artillery and air force. There will be no more talks. Everyone who fails to leave the city will be destroyed. It's up to you to choose. The countdown is already on."

The joint messages seemed designed to shift blame for civilian casualties onto the city's residents. Russia has come under intense criticism from abroad for the toll on noncombatants, who have endured three months of bombing throughout Chechnya.

The leaflet directed at civilians advised them to leave by way of a safe northwest passage through the town of Pervomaiskaya. It was not clear that the corridor was open today. Russia has occasionally closed roads out of Chechnya for seemingly bureaucratic reasons. Last week, the Russians closed the main refugee route west to neighboring Ingushetia for a day because their computers were broken; in October, the route was shut for more than a week because the computers were late arriving, stranding thousands of civilians.

More and more refugees are fleeing south into mountain towns and hamlets, and with each Russian advance, they are further cut off from an exit. Russian planes have been bombing the mountainous roads south to Georgia, according to reports from Human Rights Watch.

Tens of thousands of Grozny residents have fled to Ingushetia. Many of those left in the capital are infirm or old, left behind to oversee family property as the Russians close in. The Russians said about 5,000 guerrillas are defending the city, moving among its scarred buildings, hiding out in basements and reinforced bunkers.

"It is obvious that Russian authorities want to declare Grozny a civilian-free area, so they [can] begin to use much more powerful bombs," defense analyst Pavel Felgenhauer wrote in the Moscow Times.

In past urban assaults, the Russians have awaited the retreat of armed fighters and then negotiated with remaining civic leaders before entering the city. This time, the objective is to trap and eliminate the rebels before going in. "The goal is to destroy the large gangs positioned in Grozny, rather than just seize the city," said a spokesman for the North Caucasus Command in charge of the war.

Still, the Russians intend to avert close combat, declared Col. Gen. Valery Manilov, the deputy army chief of staff. "No frontal assault on the city is planned."

The taking of Argun testifies to Russian caution. Unlike last month's taking of Gudermes, farther east, there has been no ceremonial raising of the Russian flag in the central square nor televised meetings with townspeople. Some reports said guerrillas carry out nighttime raids on Russian positions.

There is little doubt that the Russians can follow through on their threat to step up bombing. They freely used heavy armaments, including Scud and Tochka missiles, in their assaults and have shown no concern for the progressive destruction of Chechnya's infrastructure and housing.

Russian officials said new high-tech helicopters will soon be used in Chechnya and strategic bombers will drop heavy explosives on the capital. Once the guerrillas are driven to the mountains, the Russians will use guided bombs to pursue them, said Deputy Prime Minister Ilya Klebanov.

Russian news reports also predicted that incendiary bombs will be dropped to kill fighters who hide in bunkers and caves.

Despite the ultimatums' reference to talks, no negotiations have taken place between the government of President Boris Yeltsin and Chechen President Aslan Maskhadov. Russian officials have said they consider Maskhadov ineffectual.

Generals hungry for victory have warned of mass defections from the officer corps should politicians stand in the way of the conquest of Grozny.

Staff writer Charles Babington in Washington contributed to this report.

CAPTION: Russian soldiers near the village of Goragorsk prepare Sunday to be dispatched to Chechnya. Yesterday, Moscow warned residents of the Chechen capital of Grozny, "Everyone who fails to leave . . . will be destroyed."