"There is no fighting going on in these areas. This is a 25km safety zone which the government has earmarked for the safety of the civilians.
 
"Even though we know the locations of their [LTTE's] guns and various equipment, we don't enter this location because we are concerned about the civilians."

Shelling accusations

Aid groups say more than 100,000 civilians are trapped between the army and Tamil Tiger fighters in the narrow coastal "no-fire zone". 

The United Nations have accused the military of shelling the area, and the Tamil Tigers of preventing civilians leaving, effectively using them as human shields.

S. Pasupathi from the World Tamil Relief Fund told Al Jazeera that government forces he was certain that the military had fired artillery into the area.

"From independent sources, NGOs working within the safety zone, we're getting news there has been a massive amount of shelling in the area.

"Local NGOs are working there, the Tamil Angels are working there. Authorities say they are biased but then why aren't they allowing independent observers to go there and view themselves? That alone proves that something must be seriously wrong," Pasupathi said.

Government denial

But Palitha Kohona, Sri Lanka's secretary of foreign affairs, told Al Jazeera that the government had given strict instructions to the military not to shell the "no-fire zone".

"We have no need to shell this area for a simple reason: the fruit is about to fall into our hands after 25 years of fighting. So why should we create sympathisers for the other side by shelling this area?

Focus: Sri Lanka
Q&A: Sri Lanka's civil war
The history of the Tamil Tigers
Timeline: Conflict in Sri Lanka
'High cost' of victory over Tigers
Caught in the middle
"When the LTTE, which has built up fortifications by using civilians, fire at government troops, they respond. And of course bullets don't distinguish between civilians and combatants They do cause injuries among civilians," Kohona said.

Al Jazeera's David Chater, reporting from a camp for internally displaced people set up by the government in Vavuniya, said he had been told stories apparently supporting claims that Tamil Tigers are holding civilians trapped. 

"One man said he had been hiding in the jungle with his four daughters trying to escape the Tamil Tigers, fearing they would take his daughters as child soldiers.

"Another man told me that the Tamil Tigers prevented him from leaving and put him in prison for two months before he managed to escape.

"So there does seem to be some veracity to the Sri Lankan government's claims that the Tamil Tigers are deliberately using the civilian population in the no-fire zone to stall the final advance," Chater said.

The LTTE has denied accusations of keeping civilians in the "no-fire zone" against their will.

The United Nations says at least 4,500 civilians have been killed in the last three months of fighting, and another 12,000 injured.

Ceasefire calls

Meanwhile, the United Kingdom said it was gravely concerned about the fighting and had sent an envoy to the United Nations for urgent talks.

David Miliband, the foreign minister, said the conflict in northern Sri Lanka threatened many thousands of civilian lives and repeated his call for an immediate ceasefire to allow civilians to leave the conflict area.

"The UN are making progress with the government of Sri Lanka towards an agreement to get support to civilians in the conflict zone and on efforts to get agreement from the LTTE to allow civilians to leave," he said in a statement.

"The prime minister's special representative, Des Browne, is travelling to New York to consult urgently with the UN."

In Paris, at least 11,000 Tamils demonstrated on Saturday to demand a ceasefire in Sri Lanka.

Several demonstrators wore T-shirts reading "Stop the Tamil genocide" and waved the red flag of the Tamil Tigers.