The Russian army was gearing up Friday for what it said could be the final strike against Chechen rebels, defying a Chechen declaration of all-out guerrilla war.

Seven thousand rebel fighters are holding out in the south of the region, mainly in the mountain range bordering Georgia, according to Russian military estimates, and the federal forces have vowed to root them out.

But observers say the task will not be easy. The rebels were quick to realize threats to step up fighting made early in the week by Chechen leader Aslan Maskadov when they ambushed two military trains, provoking one of the most violent clashes with Russian forces since the withdrawal from the capital, Grozny.

Russian government spokesman Sergei Yastrzhembsky told a news conference the rebels retreated after a fight when they mined a train track near the town of Argun, AP reported. He did not give casualty figures.

The attack came after Maskhadov said in a video-taped interview obtained by Reuters that large-scale guerrilla war would be launched "in the mountains, the lowlands, in every village, wherever we can."

"The people are rising up, the people have understood that [the Russian campaign] has nothing to do with the struggle against banditry and terrorism," he said, according to Reuters.

Russian forces have been trying to prevent the estimated 3,000 rebels who abandoned Grozny from meeting up with guerrillas already entrenched in the mountains. Interfax said Russian helicopters and planes flew about 200 sorties Thursday in a bid to end rebel resistance.

Alexander Pikayev, a military analyst for the Moscow-based Carnegie Center think tank, said about 40-60 sorties a day were flown in the early stages of the campaign in Chechnya.

"Two hundred sorties seems quite unprecedented and shows an intensity linked to the fact that Russian aviation is now dealing with mountain terrain that is much better protected from air strikes," he said.

Pikayev said the military looks set to continue that level of attack, given that it now seems to have sufficient funding for the operation.

But the Russians could suffer from the hit-and-run guerrilla tactics that proved successful in 1996 during the previous war, analysts say. The Russian army has pledged not to allow a repeat of that situation but has admitted it cannot always stop rebel infiltration into the areas it controls.

Pikayev said Russia may be able to stop large bodies of rebels from invading federal-held territory but would have difficulty dealing with guerrilla operations carried out by individuals who stay in their homes during the day and strike at night.

As the war rages on, both sides face problems, Pikayev said. The Russians lack an exit strategy, while the rebels lost some of their top commanders when retreating from Grozny – without achieving any real result from holding the city for so long, he said.