BBC Correspondent David Jamieson: The rebels appear to be intent on destroying Freetown

Many residents of the capital, Freetown, have been unable to venture out to obtain food and water since rebels entered the city five days ago.

Paul Ares, West Africa regional manager for the World Food Programme, said: "We are gravely concerned about the severe hardship imposed on the civilian population."

Residents of the city have been forced into hiding by militiamen.

With the city's main electricity supply in rebel hands, the population has been left in the dark.

The WFP has supplies of food ready to deliver to the city's trapped residents, but it is considered too dangerous to enter.

The starvation warning came as efforts continued to drive rebel forces out of Freetown.

There are unconfirmed reports of advances in the city by the Nigerian-led West African intervention force, Ecomog, who are backing the democratically elected government.

The rebel Revolutionary United Front - which is reported to be using civilians as human shields - says that it is maintaining the upper hand.

Mark Doyle in Guinea: Saw smoke rising as he left Freetown

The BBC's West Africa correspondent, Mark Doyle, says Ecomog forces have advanced against the rebels but are going slowly, house to house, to avoid casualties in their own ranks and among innocent civilians.

As he flew out of Freetown airport, he said a huge pall of smoke covered the city.

Rebel soldiers are reported to have set fire to the United Nations military observers' headquarters, the telephone exchange and the City Hall, among other buildings.

Our correspondent says the Nigerians are widely seen by Freetown residents as their only hope of surviving death by fire or murder at the hands of the rebels.

Ecowas diplomacy

Ecomog forces are said to be pushing the rebels back

The West African economic association, Ecowas, said Ecomog was pushing back the rebels. It said Ecomog troops, boosted by reinforcements, had taken control of the State House a week after rebels had first entered the city.

Ecowas said it was continuing attempts to bring the Sierra Leone Government and rebels to the negotiating table.

Speaking to the BBC World Service's Newshour, Ade Obisesan of Ecowas said the organisation's aim was still to hold talks between President Kabbah and rebel leaders.

Ade Obisesan: "We all want to achieve peace"

He said talks could include Foday Sankoh, the rebel leader who was jailed for treason in October - the catalyst for the current rebel action.

"The aim (of Ecowas) is to talk to whoever represents the interests of the rebels," said Mr Obisesan.