BAGHDAD, Sept 18, 2008 (AFP) - Seven American soldiers were killed on Thursday when their transport helicopter crashed in southern Iraq, the deadliest such incident in the country for over a year, the US military said.

The CH-47 Chinook went down, killing all on board, about 100 kilometres (60 miles) west of the main southern port city of Basra as it formed part of a four-aircraft convoy flying from Kuwait to the northern city of Balad.

'The total casualties for the Ch-47 is seven. This includes the crew,' Major John C. Hall said. 'All seven died as a result of the crash.'

The incident comes just two days after US General Raymond Odierno took over as the new commander of US-led forces in Iraq from General David Petraeus.

British military spokesman Major Paul Smyth said a British quick reaction force was despatched from Basra and a British road convoy in the vicinity was diverted to the scene of the crash.

He said both British and US personnel were at the scene.

The US military said the incident was under investigation. 'However, enemy activity is not suspected,' it added.

'It is a tough day for the coalition and we are deeply saddened by the loss of our soldiers,' said Colonel Bill Buckner, a spokesman for the US military.

Earlier this week, the Baghdad international airport shut intermittently as sand storms engulfed the region and reduced visibility to near zero. Hundreds of passengers travelling to and from Baghdad were stranded for two days.

Thursday's crash was the worst since August 2007 when a US Blackhawk transport chopper crashed in northern Iraq, killing 14 US military personnel.

Two American soldiers were hurt when their helicopter crashed south of the Iraqi capital Baghdad in June this year.

The worst single crash was on November 15, 2003, when two Blackhawks collided near the main northern city of Mosul, killing 17 soldiers.

In January and February last year there was a spate of crashes, including some aircraft shot down by an insurgent cell which deployed anti-aircraft crews north of Baghdad. US forces have since hunted down those gunners.

The latest deaths bring to 4,168 the number of US soldiers killed in Iraq since the 2003 invasion, according to an AFP count based on independent website icasualties.org.

The total number of US troops killed this month was 17 compared to 23 killed in August and 13 killed in July, the month with the lowest toll since the invasion. Sixty-six troops were killed in September 2007.

The military's losses have been declining in the past few months on the back of an overall fall in violence across the country.

Meanwhile, the US military arrested an American soldier serving in Iraq over the killing this week of two comrades, the military said in a separate statement on Thursday.

Staff Sergeant Darris Dawson, 24, and Sergeant Wesley Durbin, 26, were shot dead on Sunday at their patrol base near Iskandariyah, just south of Baghdad, the US military said in a separate statement.

'A US soldier is in custody in connection with the shooting deaths,' it said. 'He is being held in custody pending review by a military magistrate.'