A military spokesman said it appeared mechanical failure was the cause of the crash of the CH-47 Chinook helicopter just after midnight.

"Right now it appears it was not hostile activity. It appears to be a mechanical malfunction," the spokesman said. He said the total number of personnel on board the helicopter was seven. The original number killed had been put at five, with two soldiers missing. The remains of those two soldiers were later found, the spokesman said.

The helicopter was part of an aerial convoy flying from neighboring Kuwait to the Balad US military air base in Iraq. The crash took place about 100 km (60 miles) west of the southern Iraqi city of Basra.

The US military in Iraq mostly uses Apache attack helicopters as well as Black Hawks, which are usually deployed to transport small numbers of soldiers around the country.

The larger twin-engine Chinooks have the capacity to carry dozens of people.