Russia has closed all border crossings in and out of Chechnya as Chechens mark the 56th anniversary of their mass deportation by Joseph Stalin.

Troops have been urged to be on their guard, as the Russian authorities are concerned that Chechen rebels may use the anniversary to launch an attack either in the capital, Grozny, or in other areas now under Russian control.

For the next few days, Chechen border crossings will remain closed. Only civilians with special permits will be able to pass checkpoints within the republic.

Federal troops sealed Grozny last Friday, to prevent rebel fighters disguised as civilians returning to the bombed-out city.

A BBC correspondent who travelled around Chechnya said the situation was calm - and people had been forbidden to commemorate the anniversary.

There were some demonstrations in neighbouring Ingushetia, with protestors holding up banners demanding that the Russians leave Chechnya.

And in Russia, police on heightened alert for terrorist attacks urged civilians to be vigilant.

Anniversary

For Chechens, 23 February is a solemn anniversary.

Fifty-six years ago, Russia's then leader, Joseph Stalin, ordered the deportation of hundreds of thousands of Chechens. He claimed they had collaborated with Nazi Germany.

Many people died in exile, and a number of Chechens today regard the mass deportations as an act of revenge for anti-Soviet rebellions staged in 1942.

Refugees in neighbouring Ingushetia planned demonstrations to mark the anniversary.

It is also, coincidentally, a Russian national holiday honouring war veterans.

Russian forces marked the occasion with a military parade at Grozny airport.

'No way out'

Russian troops on Wednesday claimed to have blockaded the strategic Argun Gorge, a key gateway to the rebels' mountain strongholds.

Click here for map of recent fighting

The defence ministry said some 20,000 troops had sealed off the gorge, the scene of fierce fighting in recent weeks.

Russian warplanes and helicopter gunships flew more than 150 combat missions over the area in the past 24 hours, the ministry said. It confirmed that a MiG-24 helicopter was lost in the fighting, and that the crew had managed to return to base unharmed.

Although the rebels managed to hold on to a village at the entrance to the gorge, Russia's Defence Minister Igor Sergeyev told Interfax news agency that the fighters "have no way out anymore".

The rebels, skilled in guerrilla techniques, have previously managed to slip through Russian lines.

Atrocities

There are fresh allegations that Russian troops massacred civilians after capturing Grozny earlier this month.

Witnesses told the BBC that as many as 82 civilians had been killed by Russian soldiers in the suburb of Aldi on one day alone.

Human Rights Watch said at least 62 people had died in Aldi, when some 100 federal soldiers went on a two-day rampage starting on 5 February - several days after Chechen rebels pulled out of Grozny.

Survivors told the group that the troops had systematically robbed and shot civilians, raped several women and thrown grenades into basement hideouts.

Russia has repeatedly denied reports that its troops have taken part in atrocities.

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