THREE Black Watch soldiers were killed in a suicide attack on a vehicle checkpoint, the Armed Forces Minister Adam Ingram said last night.

Mr Ingram told a news conference at the Ministry of Defence that an Iraqi civilian interpreter had also been killed in the incident and eight other soldiers injured.

They were the first combat casualties to be suffered by the regiment since its controversial deployment to support US troops around the Iraqi capital Baghdad.

Mr Ingram said the troops had been targeted by a "vehicle-borne improvised explosive device" in a suicide attack.

Shortly afterwards, the position came under mortar fire.

The attack, at about 1pm local time, came as British troops crossed for the first time into the dangerous territory east of the River Euphrates, which had marked the boundary of their deployment.

The east bank of the river had been controlled by the US marines, but was turned over to the Black Watch after repeated rocket attacks on their Camp Dogwood base launched from the far side of the river.

The Ministry of Defence said the incident was not linked to an overnight ambush in which a Warrior armoured troop carrier was hit by a roadside bomb and a second mortared and forced off the road.

The incident threatened to reignite the bitter controversy over the deployment of the 850-strong Black Watch battle group to relieve US forces preparing for the expected all-out assault on the insurgent stronghold of Fallujah.

Liberal Democrat leader Charles Kennedy, whose party opposed the Iraq war, said it was an "extremely worrying and alarming time" for the families of the Scottish regiment.

Scottish Nationalist Party leader Alex Salmond was more outspoken, bitterly denouncing the "duplicity" of the ministers responsible for agreeing the deployment.

"There will be shock and grief across Scotland," he told Sky News. "But that will give way to a feeling of anger as we contrast the bravery of our soldiers with the duplicity of the politicians who sent them there."

It is understood that none of the survivors injured in the suicide attack was seriously hurt.

The attack underlined the dangers facing Black Watch soldiers. They have faced daily rocket and mortar attacks since they began arriving at Camp Dogwood, 25 miles south-west of Baghdad, last week.