COLOMBO - The U.N.'s aid chief told Sri Lanka's president on Tuesday of the world body's concern over casualties and humanitarian needs among civilians trapped in the last pocket of Tamil Tiger resistance to government forces.

Sri Lanka this week ordered troops to stop using heavy weapons against the Liberation Tigers of Tamil Eelam (LTTE) rebels, renewing a pledge the government had made before.

The military says it has deployed special forces, commandos and snipers using only small arms in what is now an operation aimed only at freeing civilians. But in a sign that battles were still raging the army said on Tuesday it had recovered the bodies of 28 rebels killed the previous day.

There are confrontations going on to rescue the civilians, military spokesman Brigadier Udaya Nanayakkara told Reuters. The rebels are resisting with mortars even (as) we are recovering LTTE bodies.

A U.N. statement said while Under-Secretary-General for Humanitarian Affairs John Holmes welcomed the government's announcement about the scaling down of combat operations ... the key was implementation in full of what had been announced.

It quoted Holmes as saying the pledge must be respected this time around to protect civilians, adding that in that context he expressed great concern at initial reports of continued shelling.

Holmes's meeting with Sri Lankan President Mahinda Rajapakse came after the U.N. official had inspected refugee camps near the war zone as part of a two-day visit.

S. Pulidevan, head of the LTTE Peace Secretariat, told Reueters by telephone on Tuesday the government had conducted shelling with heavy weapons and killed 75 civilians, charges the military denied.

Checking claims from the battle zone -- where 50,000 troops surround an estimated few hundred to few thousand remaining rebel fighters and tens of thousands of civilians -- is difficult given lack of access and of independent sources on the ground.

NO SURRENDER

The rebels have vowed no surrender in their fight for a separate state for Sri Lanka's Tamil minority, a struggle that began in the early 1970s and erupted into civil war in 1983.

The conventional war's impending end will leave Sri Lanka facing the challenges of healing years of division and boosting an economy beset by a declining currency, falling exports of tea and garments and low foreign exchange reserves.

It is seeking a $1.9 billion International Monetary Fund loan and business executives are optimistic the war's end will bring foreign investment back, but the LTTE has warned it will stage guerrilla attacks on economic targets as it has done before.

In its Monday announcement the government had said combat operations had been concluded and troops ordered not to use heavy-caliber guns, combat aircraft and aerial weapons.

Since the military had already been saying it was concentrating on freeing civilians in operations using only light weapons, U.S.-based rights group Human Rights Watch argued the announcement conceded that had not actually been the case.

By finally admitting it has been using heavy weapons all along, the Sri Lanka government has shed light onto its official deception as well as its brutal military tactics, said Brad Adams, Human Rights Watch Asia director. He called for an international commission to look at abuses by both sides.

Rajiva Wijesinha, chief of the government's Secretariat for Coordinating the Peace Process, responded by saying that while the military had been avoiding using heavy weapons against civilians, previously it had not eschewed the use of heavy weapons in defense.

The military has consistently denied accusations from the LTTE, United Nations and others it shelled civilians. The Tigers deny accusations they are holding civilians as human shields.

More than 113,000 have fled the battle zone since troops a week ago blew up an earth barricade blocking access, joining at least 72,000 displaced by earlier fighting. But most sources agree tens of thousands of civilians remain inside.

An internal U.N. tally of casualties says nearly 6,500 people have been killed in fighting since late January. The world body estimates over 50,000 remain in mortal danger.

Analysts say Sri Lanka's Monday announcement appeared designed to blunt diplomatic pressure for a ceasefire, which the government has ruled out given the LTTE's history of using breaks in fighting to rearm and its rejection of two truces this year.