By Mark Heinrich

CERSKA, Bosnia, May 29 (Reuter) -- A team of U.N. war crimes investigators dug up human remains on Wednesday at a suspected mass grave in eastern Bosnia where Moslems are believed to have been executed last year by Serb forces.

The six investigators are searching for evidence to convict Bosnian Serbs allegedly involved in the massacre of at least 3,000 unarmed Moslem men from Srebrenica, a U.N.-declared "safe area" overrun by the Serbs last July.

In a remote mountain forest some 10 km (six miles) from the Bosnia-Serbia border, the team partially excavated the suspected mass grave, carefully digging three trenches which revealed human remains in each case. After two hours of digging with shovels, picks, rakes and hoes, investigators allowed reporters to approach the site, where at least four corpses were found.

In the first trench, reporters saw a corpse lying on its side with the skull and pelvic bone easily visible, covered with a red-mesh shirt and what appeared to be a black sweater or light jacket. In a plastic bag alongside the corpse were a number of finger and hand bones that investigators recovered.

In a second trench three yards (metres) away, investigators unearthed what appeared to be a pair of bodies. In the third trench, reporters saw a skull with hair and skin fragments and a bullet-sized hole through it. U.N. investigators refused to speculate as to what caused the hole.

The four bodies were considerably decomposed -- mostly bones with some hair and some skin still attached. The team also marked roughly 50 spent shell casings on a slope nearby with yellow tags. The bodies were believed to be victims of the Serb conquest of Srebrenica, a Moslem enclave which fell to Bosnian Serb forces last July in an offensive that sparked international outrage.

Human remains found by the investigators could prove crucial in supporting verbal evidence from survivors who are due to testify before the U.N. war crimes tribunal in The Hague. Apart from the 3,000 Moslems believed killed after surrendering to the Bosnian Serbs, thousands of others are still missing, their fate unknown.

The Hague tribunal has accused Bosnian Serb leader Radovan Karadzic and army commander Ratko Mladic of orchestrating the slaughter of Moslem men from Srebrenica.

John Gearns, an American forensic expert leading the team, told reporters investigators would gather further evidence later but would not say when they would return. "We will put plastic sheeting on all the sites and cover them with some dirt to discourage scavengers," he said. Gearns probed the ground with T-handled metal rod and sniffed.

U.S. troops in the NATO-led peace Implementation Force (IFOR) stood in freezing rain to provide security for the team after checking the site earlier in the day for mines. An American soldier told reporters IFOR had obtained unspecified "cooperation" with the local Serb authorities for the U.N. team to examine the site. Earlier on Wednesday, the team examined a field 25 km (16 miles) northwest of Srebrenica and left after a half hour.