There were also reports of troops shooting detainees, the London-based human rights watchdog said in a report based on interviews with former prisoners of the Americans across Iraq.

Amnesty staff heard complaints that included prolonged sleep deprivation and detainees being forced to stay in painful positions or wear hoods over their heads for long periods.

"Such treatment would amount to 'torture and inhumane treatment' prohibited by the Fourth Geneva Convention and by international human rights law," Amnesty said.

U.S. military officials were not immediately available to comment on the report.

Amnesty staff gathered testimony from former detainees around Iraq and from relatives of some still being held.

The organization made several requests to visit detention centers but were denied access by U.S. forces that have struggled to impose law and order since the invasion which toppled Saddam Hussein in April.

"Detainees continue to report suffering extreme heat while housed in tents; insufficient water; inadequate washing facilities; open trenches for toilets; no change of clothes, even after two months' detention," Amnesty said.

Amnesty has said thousands are held in prisons run by U.S. troops. They include Abu Ghraib, one the most feared jails under Saddam, and Camp Cropper near Baghdad's airport.

The human rights group said it had received several reports of cases of detainees who have died in custody, "mostly as a result of shooting by members of the coalition forces."

Amnesty said 22-year-old Alaa Jassem was killed when soldiers fired on detainees during a riot on June 13 at Abu Ghraib. Demonstrators threw bricks and poles at the soldiers.

"According to eyewitnesses, Alaa Jassem was in a tent when he was shot. Seven other detainees were wounded," Amnesty said.

Other allegations reported by Amnesty included the case of Saadi al-Ubaydi on the morning of May 14, when two U.S. armed vehicles crashed through the stone wall surrounding his home.

"Several soldiers forced their way in and beat him with their rifle butts. He ran out of the house to get away from them. Soldiers shot him a few meters away and he died immediately," the report said, citing witnesses in Ramadi.

Many Iraqis complain troops use heavy-handed tactics that humiliate householders when conducting weapons searches.

"There continue to be many reports of members of the coalition forces engaging in house searches and damaging or destroying property without justification," Amnesty said.