The US military has confirmed it is investigating allegations that members of a reserve US army unit in Iraq refused to undertake a convoy mission.
US convoys have been a main target of attacks by insurgents
The unit involved is responsible for transporting food, water and fuel for US-led forces, a top US official said.
Up to 19 soldiers from the unit based near Talil in southern Iraq allegedly failed to carry out their orders.
The news came amid reports that UK troops could be sent to back up the US in some of Iraq's most volatile areas.
The UK Ministry of Defence said on Friday that discussions were under way but no decision had been taken as yet.
It is believed the troops could be sent from Basra in southern Iraq to an area south of Baghdad, and might - controversially - be under US command.
In other developments in Iraq:
A mortar explodes outside a Baghdad hospital killing one member of staff and injuring several others
- A wave of attacks hits at least five Christian churches in the capital Baghdad
A US soldier dies of wounds sustained in a car bomb attack on a convoy in the northern city of Mosul on Friday
The UN says Thursday's bombings in Baghdad's Green Zone have underlined its concern about security in Iraq.
Families of the US reservists being investigated are reported to have said the troops considered the mission too dangerous.
At least some have been quoted as saying they refused the mission because their vehicles were in poor condition and they did not have an adequate armed escort.
Teresa Hill of Dothan, Alabama, told the Associated Press news agency that her daughter Amber McClenny who serves in the platoon had phoned on Thursday morning to ask her to help.
"This is a real, real big emergency," Ms McClenny said. "I need you to contact someone. I mean, raise pure hell."
"We had broken-down trucks, non-armoured vehicles and ... we were carrying contaminated fuel.
"They are holding us against our will. We are now prisoners," she added.
But according to a senior US military official, the unit involved had been ordered to carry out what is known as a maintenance stand-down.
The official said the soldiers involved were not under arrest or detained and he described the incident as isolated.
However, a US-led coalition spokesman in Baghdad told AP that a few of the troops had chosen to express their concerns in an "inappropriate manner" and caused a temporary breakdown in discipline.
The BBC's Nick Childs in the Pentagon says US convoy missions have been a main target of attacks by Iraqi insurgents, but are also vital to the US-led forces - so any breach of military discipline here is likely to be taken seriously.