In some streets the opposing sides are just metres apart, shouting insults at each other as they exchange fire, Russian television reported.

About 2,000 rebels are thought to be defending the city, which is under attack from several directions.

The Russians have deployed huge numbers of ground troops in the assault, and are calling in air strikes on Chechen positions whenever they encounter serious resistance.

Chechen sources have confirmed that their defence lines in parts of Grozny are under heavy attack from Russian tanks and constant air bombardments.

A spokesman for Chechen President Aslan Maskhadov told the French news agency AFP that the key Zhukovsky bridge linking east and west Grozny had fallen to the Russians.

Click here for a map of Grozny

However the Chechens say they are not ready to surrender their capital. The rebel fighters have spent months digging in and preparing for the battle.

Russian military commanders are predicting the onslaught will break the rebels and that the city will fall in days.

Late on Tuesday, fires were burning all over the city and a thick pall of smoke was blocking out the light.

The Russian military told the Itar-Tass news agency that fighting had continued unabated after nightfall.

It said federal forces were shelling Chechen positions from the high ground on the perimeter of the city.

A BBC correspondent reporting from Russian military headquarters in Mozdok says Grozny, which is systematically being ground to dust, has become a sniper's paradise - a wasteland of rubble and hollowed out buildings.

'Decisive phase'

The Russian military says it now controls at least three districts and has dislodged Chechen fighters from their positions around Minutka Square in the city centre.

"The decisive phase of the liberation of Grozny has started," said Defence Ministry spokesman Konstantin Kukharenko.

The United Nations High Commissioner for Refugees has expressed renewed fears for the safety of thousands of civilians still trapped in Grozny, but says it is ''powerless'' to help.

The UN refugee agency said many of those stranded in Grozny were too old or too frail to leave, and were hiding in damp cellars hoping to survive the onslaught.

But UNHCR spokesman Kris Janowski said it was too dangerous to send in relief workers.

He said Russian sources had indicated that about 12,000 civilians were still in the city. But others put the figure as high as 20,000.

"The humanitarian agencies are basically incapable of helping these people, so they are stuck there without humanitarian aid and we don't even know what's happening with them."

Bombardment

Russian warplanes have also struck the Vedeno and Argun gorges, which lead through rebel-held mountains in the south to the republic of Georgia, while helicopter gunships rocketed Grozny and southern towns still occupied by the separatist fighters.

The Russian paratroops commander said federal forces had blocked off all key towns and villages around the gorges.

Moscow moved troops into Chechnya last September after blaming separatist militants for incursions into Dagestan and bomb attacks in Russian cities.

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