COLOMBO (REUTERS) - Officials investigating a suspected mass grave in Sri Lanka's former northern war zone called off the digging on Friday because the 83 skeletons unearthed there seem to have been buried in an old cemetery.

The Sri Lankan army has been accused of killing tens of thousands of civilians in the final weeks of its 26-year war against northern Tamil separatists in 2009 and Colombo has been under pressure to investigate reports of mass graves there.

Police at first suggested the Liberation Tigers of Tamil Eelam (LTTE) separatists could be responsible for the burial site near a Hindu temple in the northwestern town of Mannar.

But Senerath Dissanayake, director general of the state-run Archeological Department, said it is not a mass grave as the bodies had been "buried systematically".

"These are about 50 years old. It is a grave yard. Even our officers can identify and see the cut marks of the graves. We have found 83 skeletons so far and we will stop (excavation)from tomorrow," he told Reuters.

More than 100,000 people were killed in the war since its start in 1983 and thousands, mainly from the minority Tamil community living mostly in the north, are still missing.

The United States plans to submit a resolution criticising Sri Lanka at this month's session of the U.N. Human Rights Council in Geneva, calling the Human Rights High Commissioner's office to investigate the charges of war-related rights abuses.

A UN panel has said around 40,000 mainly Tamil civilians died in the ferocious final months of the conflict, but Sri Lanka has disputed that figure. Both sides committed atrocities, but army shelling killed most victims, it concluded.

Dhanajaya Waidyaratne, the judicial medical Officer in charge of the excavation, described the bottom of the suspected pit as "an area which looks like an old cemetery".

"We excavated up to 2 meters depth. We have not started any forensic tests. That is still pending," he told Reuters, adding the bodies had been buried in layers.

Three area residents told Reuters in January there was no cemetery in the area from the time they moved there around 1970.

Residents say the area was controlled mainly by the army from 1990 but the military has disputed the claim, saying it was under the LTTE control during the most of the war period.

Mannar's Catholic Bishop Rayappu Joseph said international experts were needed because residents had no confidence in the Sri Lankan investigators.

"They have made a conclusion on their own. It is very disgraceful for the dead bodies. We need a justifiable probe and acceptable conclusion," he said.

"These bodies could be sent to the US, China and Europe for forensic reports to find the cause of death," he said.

Nine bodies, most of them women, were found last week in a mass grave near where the war's last bloody battle was fought.

Last year, Sri Lanka set up a presidential commission to investigate a mass grave with remains of more than 150 people found in a central province. Evidence was sent to China for forensic investigations and so far there has been no conclusion.

Speculation is widespread in the north that there may be many more such graves containing the remains of thousands who went missing during the war.

UN human rights high commissioner Navi Pillay urged Sri Lanka last week to accept international help to investigate grave sites promptly and properly.

"The rights of families to know the fate of their missing loved ones is critical," she said in a report. "An uncompromised exhumation and investigation process is essential, and could benefit from international assistance."