NEW DELHI - THE Sri Lankan government has won its war against the Tamil Tiger rebels, but must now battle to ensure it does not lose the peace.

The chorus for an international probe into alleged war crimes is gathering steam, even as aid agencies charge that too little is being done to rush help to affected civilians.

The prospect of war-crimes charges loomed larger yesterday, with United Nations chief Ban Ki Moon backing growing calls for a full investigation.

Thousands of civilians are believed to have been killed during the offensive against the rebels, which climaxed on Monday with the killing of Tiger chief Velupillai Prabhakaran and the overrunning of the last shred of insurgent-held territory.

Both sides have been accused of rights violations, with aid bodies and rights groups alleging indiscriminate army shelling and condemning the Tigers for using civilians as human shields.

The UN Secretary-General, who is due to visit Sri Lanka at the end of the week, said in Geneva on Tuesday that any serious allegation of war crimes "should be properly investigated".

'I remain concerned about the welfare and safety of the civilian population,' he added.

The UN's main rights body is to hold a special session on Sri Lanka next week.

Even now, however, Colombo has severely restricted access to the 265,000 non-combatants who have escaped the fighting, despite widespread fears of a deepening humanitarian crisis.

The UN Refugee Agency said the government urgently needed to allocate more land for emergency shelter, water and sanitation facilities. The agency said its access to civilian camps in the district of Vavuniya had recently been "greatly curtailed" and warned of another exodus of tens of thousands of people in the coming days.

The International Committee of the Red Cross, the only neutral group that was working in the conflict zone, said the military's final assault had halted its access to desperate civilians.

'No humanitarian aid has reached those who need it for over a week,' director of operations Pierre Kraehenbuehl said.

Former colonial power Britain also wants a probe, citing a "truly shocking and appalling" civilian death toll.

But though UN officials described the final days of the fight as a "bloodbath on the beaches", the hawkish government of President Mahinda Rajapakse called its offensive the "world's largest hostage-rescue mission".

It has not accepted blame for a single civilian death.