COLOMBO: A pro-Tamil Tiger website Tuesday accused Sri Lankan government forces of continuing to pound civilians trapped in rebel-held territory despite a pledge to stop using heavy weapons.

The military immediately denied the allegation, but confirmed it was trying to capture more territory and free the civilians it says are being held as human shields by the outnumbered and encircled rebels.

"There was heavy mortar fire from many directions," Tamilnet reported, adding that 139 people were wounded in attacks Monday evening -- hours after President Mahinda Rajapakse's office pledged that air strikes and attacks using heavy-calibre weapons would stop.

Sri Lankan military spokesman Brigadier Udaya Nanayakkara said fighting was continuing -- despite international calls for a ceasefire to spare civilian lives -- but denied the army was firing indiscriminately.

"We did not shell the area. We have not used heavy weapons against civilian areas even before," he said. "But ground operations to capture territory and rescue civilians will continue."

The defence ministry said troops were advancing, after another layer of rebel defences -- leaving the Liberation Tigers of Tamil Eelam (LTTE) with just six square kilometres (2.5 square miles) of land.

"The earth bunds (embankments) were heavily mined, made to restrict both flow of the remaining hostages and military advances," the ministry said.

On Monday, the United Nations' top humanitarian official John Holmes welcomed Colombo's promise to scale down its assault but said he was disturbed by reports of continued shelling.

"I hope it (the government declaration) will be genuinely respected this time," Holmes said. "It has not happened in the past."

A United Nations document circulated among diplomats in Colombo last week said as many as 6,500 civilians may have been killed and another 14,000 wounded in the government's offensive this year.

The United Nations estimates that some 50,000 non-combatants are still trapped in the conflict area, while the government maintains that the number is fewer than 20,000.

The Sri Lankan government says its forces have cornered the Tamil Tigers in a small patch of coastal territory in the district of Mullaittivu, but has come under pressure over its conduct of the war.

Holmes left Colombo late Monday having failed to secure an agreement from the Sri Lankan government to open up the conflict area for humanitarian aid and relief workers.

He also said the UN was pressing the government to ensure that the civilians are held in camps in line with international standards and allowed freedom of movement which is currently denied to them.

The island's government has for months blocked most aid agencies from working in the war-torn north, and has herded escaping Tamil civilians into closely guarded internment camps so it can weed out suspected rebels.

Pressure on the government is set to mount this week, with Britain and France sending their foreign ministers to the island.

Diplomats said British Foreign Secretary David Miliband and his French counterpart Bernard Kouchner, who are due in Colombo Wednesday, would press for a lifting of the ban on aid workers accessing the conflict area.

The ministers are expected to travel to Vavuniya -- where Tamil civilians have been placed in screening camps -- but were not expected to be taken close to the conflict area.