Russian federal forces launched a key stage of an operation to storm Grozny, capital of the breakaway republic of Chechnya, Friday midnight, the Itar-Tass news agency reported Saturday.

The goal of this stage is to take full control of Grozny and to clear the city of rebels, Itar-Tass quoted credible sources at the staff of the federal army grouping as saying.

A 500-member force of Chechen volunteers headed by Grozny's former mayor Beslan Gantamirov, is fighting a way into central Grozny from three directions, said the sources.

An Interfax report citing Chechen sources said that intensive fighting has been underway in Grozny since Saturday morning.

Federal soldiers of the Western and Eastern groups are advancing from the city outskirts and tightening their encirclement of the Chechen rebels with heavy artillery pounding the city throughout the night and morning, said the report.

While explosions are heard "almost every minute" and large- caliber weapons are firing from all directions, the Russian troops are using smoke as a screen to keep the rebels from being able to fire accurately at their armored vehicles, said the report.

According to Russian military sources, a tactic proven effective during previous operations is being used in the ongoing operation.

Grozny has been mapped into 15 sectors in which the federal airpower and army reconnaissance units have scouted rebel positions.

The airpower and artillery units shelled strongholds, military hardware and fortified positions of the rebels.

Sapper teams supported by mortar fire and snipers will clear paths of land-mines for Gantamirov's units.

The plan of the ongoing operation envisions a "web-like" pattern of the presence and action of federal troops in Grozny.

After Grozny is under the "web," the third stage of the operation will start, which is designed to destroy the rebel manpower.

Under the plan, the "web" is to deprive rebels, who fight in small groups, of their best asset of mobility.

The sources said that it could take the federal units a few days to fly the Russian flag on Chechen President Aslan Maskhadov's residence on Street Revolyutsiy in downtown Grozny.

Russia began its military offensive against the Chechen separatists in late September after Chechen rebels' armed incursions into neighboring federal republic of Daghestan and a spate of apartment building blasts that claimed over 300 people in Moscow and two other Russian cities. Russian authorities blamed Chechen extremists for planning and carrying out the terrorist acts.

Earlier, Russian groupings successively liberated the second biggest Chechen city of Gudermes and other key towns of Argun, Urus-Martan and Shali, from rebel control, and closed the surrounding of Grozny. So far, Russian units have reportedly controlled all flat lands of Chechnya, or over two thirds of the republic's territory, where 90 percent of Chechen population live. (Xinhua)