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.

A second train, which came to the rescue of the first, was also damaged when the rebels launched rocket-propelled grenades at its engine.

The attackers retreated after Russian special forces arrived, according to the Interfax news agency.

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

Correspondents say the ambush demonstrates that bands of well-armed Chechen rebels are still active, even in areas that Russian forces claim to control.

Federal troops took control of Argun, 15km (9 miles) east of Grozny, in December, but rebels managed to retake parts of the town for a few days in early January before again abandoning it.

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.

Mountains

Russia is meanwhile reported to be carrying out heavy air strikes on rebel strongholds in the mountains of southern Chechnya.

The Russians say they are trying to wipe out the bases of a remaining 7,500 Chechen fighters.

Russian estimates speak of as many as 60,000 troops being poised for a final assault.

Interfax reported heavy fighting on Thursday in the village of Katyr-Yurt, where the military believed a rebel field commander was holed up with a group of militants.

Katyr-Yurt is located about 30km (18 miles) southwest of Grozny.

Interfax said Russian helicopters and planes had flown about 200 sorties over the past 24 hours, more than double the average, with federal troops now poised for a final assault.

Chechen rebels use the Argun gorge, and the Vedeno gorge further east, as lines of supply.

Russia says it controls the entrances of both gorges.

Russian paratroopers have also been dropped at the top of the Argun gorge.

Chechen rebels have been blowing up paths to prevent the paratroopers coming down the valley, the Itar-Tass news agency reported.