Russian war planes have been pounding rebel positions in southern Chechnya as Moscow prepares to send 50,000 troops into the mountains to pursue Chechen rebels.

According to Russia's North Caucasus headquarters, Russian bombers and helicopter gunships have been carrying out hundreds of raids in the Argun and Vedeno gorges south of Grozny.

The Kremlin said Russian troops had seized the rebel stronghold of Itum Kale in the Argun gorge.

The Chechen leader, Aslan Maskhadov, has warned that his forces are beginning an all-out guerrilla war against the Russians.

The Interfax news agency reported heavy fighting in the village of Katyr-Yurt, where the military believed a rebel field commander was hiding with a group of militants.

Russian troops have also begun pulling out of Grozny which fell to them on Sunday.

But there are reports that Russian officers are dreading the mountain campaign.

They are under pressure to stop Chechen fighters rearming and regrouping themselves as they did in the first Chechen war in 1994-96.

One soldier, Gennady Vasilyevich said, "It is not possible to get them out of the mountains that easily."

"We were not able to do this for two years during the last war."

A BBC correspondent travelling inside Chechnya says the landscape bears the scars of vicious battles. He said that in one village the sky was darkened by the smoke of a burning oil refinery, buildings were shattered and the people humiliated.

Troop cut

Russia is planning to cut its forces in Chechnya by nearly half, a senior military official has confirmed.

"There will be about 50,000 troops left, and they will be mainly paratroops, marines and rapid reaction forces," Valery Manilov, first deputy chief of Russia's General Staff, told ORT television.

"They will be people trained to fight in mountains," he said. "Enough troops will remain to deal with the situation."

Last week, Russian officials said the force in Chechnya numbered 93,000.

Train ambush

Meanwhile, Chechen rebels have ambushed a military train in a Russian-controlled region of Chechnya, the Russian defence ministry says.

The train was being used to repair railway tracks damaged by fighting near the town of Argun.

Rebels detonated three remote-controlled mines under its locomotive and opened fire from four directions, the Itar-Tass news agency reported.

None of the railway guards are reported to have been injured in the attack, which happened on Tuesday. Details have only now been disclosed.