An excavation is underway of what may be the largest mass grave of ethnic cleansing victims in Bosnia.

The existence of a site was known for years but the exact location was covered up by a Serb wall of silence, the state's missing persons office said.

The U.N. war crimes tribunal in The Hague has sentenced 16 Bosnian Serbs to a total of 230 years in prison for atrocities in the Prijedor area of northwest Bosnia where the Tomasica site is located. But not all mass graves have been found.

At Tomasica, forensic experts have been recovering bodies of Bosniaks (Bosnian Muslims) and Croats killed by separatist Serbs in one of the worst ethnic cleansing campaigns aimed at driving out non-Serbs in the country's war.

On Tuesday, the remains of 18 people were unearthed from the grave, which spreads across 3,000 square metres in uninhabited green hills. All the victims, men, women and children, were shot dead, forensics workers said. Many were wrapped in blankets.

The first bodies at Tomasica were found at the depth of seven metres (23 feet) in August and the remains of 240 people have been exhumed to date - believed to be Muslim villagers from the Prijedor area.

Witness accounts have established that around 1,000 people were buried there originally but there are indications that some were subsequently dug up and reburied elsewhere to cover up traces of the crime.

In 1992, Bosnian Serbs set up four detention camps around Prijedor for around 7,000 Muslims and Croats. They were tortured, malnourished and many eventually killed, war crimes investigators said.

Tomasica is one of 445 grave sites, including almost a hundred mass graves, found in northwest Bosnia since the war.

The Prijedor region was a stronghold of ultra-nationalist Serbs during the war who sought to oust all Bosnian Muslim and Croat inhabitants to create exclusive Serb territory in Bosnia. Nearly 3,500 people went missing from Prijedor town alone.

Some 1,200 bodies of the victims are still being sought.