NEW DELHI: The prospect of war crimes charges following Sri Lanka's military victory over Tamil Tiger rebels loomed larger Wednesday, with UN 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 Liberation Tigers of Tamil Eelam (LTTE).

Both sides in the conflict have been accused of rights violations, with numerous aid agencies and rights groups alleging indiscriminate army shelling and condemning the Tigers for using civilians as a human shield.

The UN secretary general, who is due to visit Sri Lanka at the end of the week, told reporters in Geneva on Tuesday that any serious allegations of war crimes "should be properly investigated." "I remain concerned about the welfare and safety of the civilian population," Ban added.

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

"The Human Rights Council cannot be silent when innocent civilians are caught up in armed conflicts," said council president Martin Ihoeghian Uhomoibhihe.

"The international community must strive to deliver justice to victims of human rights violations," he added.

The military declared final victory in the decades-old conflict on Monday after overrunning the rebels' last holdout in the northeast.

In the run-up to the final battle, there was a stream of eyewitness testimony to shelling by government troops that caused significant civilian casualties -- testimony that has been staunchly denied by the military.

Aid workers were also denied access to those trapped by the fighting, despite accounts of a jungle area littered with the dead or dying, with the elderly, women and children cowering in shallow dug-outs with little food or water for several months.

"There has to be accountability for what has gone on in Sri Lanka, there has to be clarity and there cannot be impunity," Rupert Colville, a spokesman for UN High Commissioner for Human Rights Navi Pillay, said last week.

Pillay said in March that both sides may be guilty of war crimes.

"Nothing we've seen since then has caused us to change our minds, quite the contrary," Colville said, adding that an independent inquiry of some form "is now essential." Former colonial power Britain also wants a probe, citing the "truly shocking and appalling" numbers of civilian dead.

British Prime Minister Gordon Brown had warned the Sri Lankan government that there would be "consequences for its actions." UN officials say more than 6,500 Tamil civilians have died in the conflict since the start of the year, with the final days of the battle described as a "bloodbath on the beaches." According to the campaign group Human Rights Watch, satellite imagery and witness accounts contradict government claims that heavy weapons have not been used. It said both sides in the war were using civilians as "cannon fodder." Amnesty International has also demanded that "the mounting evidence of serious violations of international law" be investigated.

The hawkish government of President Mahinda Rajapakse described its military operation against the Tamil Tigers as the "world's largest hostage rescue mission," and has yet to acknowledge blame for a single civilian death.

It is also convinced that any criticism of its handling of the war has been either an effort to save the rebels or blatant hypocrisy.

"They are trying to preach to us about civilians," Rajapakse said at a recent public rally after meeting British and French ministers. "I tell them to go and see what they are doing in Iraq and Afghanistan."