American soldiers tortured Iraqi prisoners by strangling them, beating them and placing lit cigarettes in their ears, even after the Abu Ghraib prisoner abuse scandal came to light in April 2004, according to new allegations.

Internal FBI memorandums made public on Monday by the American Civil Liberties Union include detailed reports of detainees in Iraq being chained for long periods, left to defecate on themselves in cold prison cells, and other inhumane treatment.

The ACLU said the documents provide a paper trail implicating senior U.S. government officials in the illegal practices carried out by U.S. soldiers.

There were also accounts of detainees being tortured at the Guantanamo Bay prisoner camp in Cuba.

The documents, mainly e-mails written by FBI agents to superiors in Washington, were released as part of a lawsuit that alleges U.S. government complicity in the abuse of Iraqi and Afghan prisoners.

The documents had been obtained under the Freedom of Information Act.

One of the e-mails, dated June 24, was addressed to FBI director Robert Mueller. It provided the account of someone "who observed serious physical abuses of civilian detainees" in Iraq.

Sent as an "urgent report" by a correspondent whose name was blocked out, the heavily edited communication testified to abuses that included "strangulation, beatings, placement of lit cigarettes into the detainees' ear openings and unauthorized interrogations."

On its website, the ACLU said the documents suggested "President Bush issued an executive order authorizing the use of inhumane interrogation methods against detainees in Iraq."

According to the documents, those methods included "sleep deprivation, stress positions, the use of military dogs, and "sensory deprivation through the use of hoods, etc."

Anthony D. Romero, executive director with the civil liberties watchdog, said that "top government officials can no longer hide from public scrutiny by pointing the finger at a few low-ranking soldiers."

Two studies commissioned by the Pentagon that reported during the fall had largely cleared senior military officers and politicians from any involvement in the scandal.

The studies said the abuses had been committed by individual soldiers, without the knowledge of high officials such as U.S. Defence Secretary Donald Rumsfeld, or the president.

13 individual soldiers have been convicted for prisoner abuse offences.

Another allegation raised in the documents is that, in a scheme approved by Deputy Defence Secretary Paul Wolfowitz, U.S. military interrogators had impersonated FBI agents, apparently to avoid blame in any subsequent inquiries.

The U.S. government would not comment beyond saying the Pentagon was investigating some of the allegations.

On Monday, the U.S. said it would soon release an unnamed Guantanamo Bay detainee, who had been wrongly classified as an "enemy combatant."