On Tuesday Phil Shiner sent Reuters statements by two Iraqis who said they were hooded, stripped naked, beaten and doused with cold water at lengthy torture sessions in a place called "the disco" because of loud Western music constantly blasted at detainees.
One said he had seen a 14-year-old boy bleeding from his anus. The other said he was threatened with sexual assault.
The allegations appear to be the first reports of abuse in Mosul.
Washington has acknowledged detainees were abused at Abu Ghraib, and Shiner is leading a case on behalf of Iraqis who say they were mistreated by British forces in Basra.
Three Iraqi staff working for Reuters say they were beaten and sexually humiliated by US troops near Falluja in January.
A US military spokesman for detainee operations in Iraq said in Baghdad he was unaware of the allegations.
"I am surprised by them," he said. "I have visited that facility up there and I've seen the good work that they've been doing."
But in the statements provided by Shiner, Haitham Said al-Mullah, an engineer, said he was brought to "the disco" where "they left me standing for hours, handcuffed and hooded".
"Then I was kicked very hard in the stomach, which was followed by continuous beating with a stick and with their boots until I fell unconscious. I only woke up after they poured over my head very cold water."
The Mosul allegations echo those
first head in Abu Ghraib prison
He said he was then abused in groups with other detainees, forced to carry out exhausting exercises and beaten or doused with cold water whenever they fell to the floor. They were not permitted to use the toilet and allowed only two hours of sleep.
"I saw a young man of 14 years of age bleeding from his anus and lying on the floor. He was Kurdish and his name was Hama.
I heard the soldiers talking to each other about this guy.
They mentioned that the reason for this bleeding was inserting a metal object in his anus," he said.
Abused by day
The other alleged Mosul victim, Yasir Rubaai Said al-Qutaji, was described as an Iraqi lawyer investigating reports of abuse at "the disco" when he was arrested.
After day and night being forced into stress positions and doused with cold water at "the disco", he was taken to a regular prison. Staff and interrogators there treated him properly at night, but allowed the same "disco team" to abuse him by day.
He was threatened with sexual assault on his final day.
"The only reason he was detained was that he was working on documenting these cases of torture, at this prison and the Americans then went and detained him," Shiner said.
The US government has said abuse of prisoners in Iraq was mainly confined to a few rogue soldiers at Abu Ghraib.
The US army says investigation
showed no evidence of abuse
But in January, three Iraqis working for Reuters said they had been subjected to sexual humiliation, anal abuse, beatings, stress positions, sleep deprivation and disorientation using bright lights and loud music during three days of detention at a US base outside the city of Falluja west of Baghdad.
The US army denied the allegations, saying an investigation found no evidence of such abuse.
The Iraqis working for Reuters were never interviewed for the investigation, which took evidence only from US soldiers who denied that abuse took place.
The Pentagon has rejected demands by Reuters for a more thorough investigation.