A letter from the head of the Defence Intelligence Agency (DIA) to a senior Pentagon intelligence official, made public on Tuesday, detailed previously unknown incidents of abuse by US forces against prisoners in Iraq.
It said the agents also saw detainees with burn marks and bruises.
It was written two months after photographs of US soldiers abusing prisoners at Abu Ghraib prison near Baghdad became public, and five months after American commanders in Iraq first learned of the abuse.
The Abu Ghraib revelations prompted international outrage and undercut US credibility as it sought to stabilise Iraq amid violence after last year's invasion.
The new revelations of abuses elsewhere were included in a 25 June letter from navy Vice-Admiral Lowell Jacoby, director of the DIA, to Stephen Cambone, undersecretary of defence for intelligence.
The letter was one of numerous US government documents released by the American Civil Liberties Union (ACLU), which obtained them under the Freedom of Information Act.
Other documents depict a split between the defence department and the FBI over Pentagon use of harsh interrogation methods on prisoners.
Threats against agents
Jacoby wrote that two unidentified DIA agents, who worked as interrogators and debriefers at a detention facility in Baghdad, saw task force officers "punch a prisoner in the face to the point the individual needed medical attention".
Iraqis were appalled by the Abu
He said that "the debriefer was ordered to leave the room". The date of the incident was not stated.
The DIA personnel also observed "prisoners arriving at the temporary detention facility in Baghdad with burn marks on their backs. Some have bruises, and some have complained of kidney pain", Jacoby wrote.
One of the DIA agents took pictures of the injuries and showed them to his supervisor in the task force "who immediately confiscated them", Jacoby added.
Members of the task force acted against the DIA agents, the letter said, including making unspecified threats, confiscating their vehicle keys, ordering them "not to talk to anyone in the US", and telling them their email messages were being screened.
The letter said task force members also "instructed them [the DIA agents] not to leave the compound without specific permission, even to get a haircut at the PX (store for military personnel)".
The job of the task force was not specified. During the Iraq war, the Pentagon has used several task forces made up of special operations troops and sometimes CIA agents, searching for "high-value" fugitives and weapons of mass destruction.
Pentagon report due
Jameel Jaffer, an ACLU lawyer, said the documents show an attempt to cover up abuse, noting the threats made to the two DIA agents.
Several soldiers were punished
after the Abu Ghraib scandal
Pointing to the account of burns on the backs of some prisoners and bruises, Jaffer added: "By anyone's definition, that suggests that something akin to torture has been going on. This is alarming."
The Pentagon had no immediate comment on the documents.
It has previously acknowledged abuse of prisoners in Iraq by special operations troops.
Members of an elite navy Seal unit were charged in September with abusing prisoners, including one who died in November 2003 after being dropped off at Abu Ghraib with severe head injuries.
A Pentagon report on detainee treatment by special forces in Iraq is due to be made public as early as next month.