A British military spokesman meanwhile said that UK troops had killed two gunmen in a raid near the border town of Safwan, close to where the convoy was attacked after crossing from Kuwait.
But the spokesman said that raid was not related to the search for the missing foreigners.
The four Americans and one Austrian who were reported missing had been guarding a large convoy that was travelling northwards from the Kuwaiti border.
The convoy was being operated by the Crescent Security Group, a company that works mostly in Iraq and has its operations based in Kuwait.
Many of its managers and employees are American. A spokesman on Friday declined to comment, saying that officials from the group were discussing the situation.
Lieutenant Colonel Christopher Garver, a US military spokesman in Baghdad, confirmed on Thursday that a convoy was attacked in the southern region of Iraq, but he provided no other details.
In Baghdad on Friday, a spokeswoman for the US military would only confirm that “an incident occurred at a checkpoint”, saying the military would probably provide more information later.
A spokesman for Britain‘s defence ministry said that no British civilians or military forces were attacked or taken hostage during the hijacking.