By Imogen Foulkes
BBC correspondent in Geneva
The UN's Human Rights Commission repeatedly asked coalition forces for explanations about deaths in detention in Iraq, it has been revealed.
The letters express concern over the treatment of prisoners
In letters seen by the BBC, senior UN official Asma Jahangir writes to the US and UK of her concern over deaths in custody and extra-judicial killings.
She lists a series of allegations about the use of disproportionate force by coalition troops.
The letters also describe reports of cruel treatment of Iraqis.
To the US ambassador to the UN in Geneva, she expresses deep concern over reports of deaths in custody as a result of what she describes as degrading and humiliating forms of torture.
The UN special rapporteur on extra-judicial killings also makes clear that she has already written to the US ambassador three times on this subject.
She asks the US to supply her with a complete list of all cases of suspected deaths in custody in Iraq and details of investigations carried out into them.
To the British ambassador to the UN in Geneva, she writes of specific allegations of extra-judicial executions carried out by the Royal Military Police.
These letters are just the latest development in the growing controversy over the behaviour of coalition forces.
The UN Human Rights Commission is due to publish a full report into the human rights situation in Iraq at the end of this month.