Seven US soldiers have been killed after their helicopter crashed as it was attempting to land in southern Iraq, the US military has said.

The CH-47 Chinook crashed just after midnight on Thursday, about 100km west of the southern city of Basra, a statement from the US military said.

Major Paul Smyth, a British military spokesman, said there were no reports of any hostile action in the area.

"The exact cause of the crash is under investigation.

"A British quick reactionary force was despatched from Basra and a British road convoy which was in the vicinity was diverted to the scene," Smyth said.

The helicopter was among an aerial convoy flying from Kuwait to the US military base at Balad, which lies north of the Iraqi capital, Baghdad.

The US military statement said that an investigation has been opened into what caused the crash.

It was the worst helicopter crash in Iraq since August 2007 when a US Blackhawk transport helicopter crashed in northern Iraq, killing 14 US  military personnel.

The Chinook model was introduced in 1962 and is primarily used to transport troops and supplies to combat and other regions.

The latest deaths bring the total of US soldiers killed in Iraq since the 2003 invasion to 4,166, according to a count by the AFP news agency.