An Iraqi soldier killed two American soldiers and wounded six more at a joint security base in the northern city of Mosul, the worst case yet of US-trained local troops turning their guns on their allies.

The soldier, identified as Barzan al-Hadidi, was part of a joint force patrolling the city, considered to be one of the last strongholds of al-Qaeda and its allies in Iraq. He was quickly shot dead by other members of the patrol, officials said.

The Iraqi Interior Ministry said that the soldier opened fire after he was slapped publicly by an American colleague. Many Iraqi men, especially in the military, are conscious of any perceived slight to their honour.

“Two soldiers were killed and six wounded in a small-arms fire attack in an Iraqi army compound in Mosul. Initial reports indicate the attacker was an Iraqi soldier,” the US military said. “The casualty figures may change,” it added. A mortuary that received the body of the Iraqi soldier said that it was riddled with bullets.

A spokesman from the US military later denied that the slapping incident had taken place.

He said: "There was no altercation between the US or Iraq soldiers leading up to the incident."

There have been instances of Iraqi soldiers opening fire on their US mentors in the past but never to such a deadly extent. Insurgents have also used Iraqi army uniforms to infiltrate bases. In Mosul in 2004 a suicide bomber dressed as an Iraqi soldier blew himself up in an American army mess hall, killing more than a dozen soldiers.

The incident yesterday highlights the dangers of joint patrols with an Iraqi Army that is still being shaped by the US military, and on to which Washington hopes to shift the burden of security as it removes its forces.

Those security challenges have loomed larger in recent days, with a spate of bombings and shootings recalling the dark years that Iraqis had hoped were behind them.

Two dozen people were killed in attacks across the country yesterday, 12 of them in a combined car bombing and roadside booby trap in eastern Baghdad. In what is becoming a familiar pattern, the first explosion was aimed at a police patrol and the second concealed device cut down those who ran to the aid of the stricken security forces. Sixty people were wounded in the blasts.

Earlier in the day four people were killed and 13 were wounded by a car bomb in the northern Baghdad area of Shaab, one of the more volatile areas of the capital. Another seven people were wounded in a separate explosion in the same area.

In the city centre yet another car bomb killed four people, including two police officers, and a car bomb in the south of the city killed four more people, officials said.

In further violence in Mosul two Christian sisters working for the local council were murdered by gunmen who broke into their home. The gunmen wounded the women’s mother before setting a booby trap made of five separate bombs that wounded two police officers when they came to the home.

The increase in violence included the use of the youngest suicide bomber to date this week: a 13-year-old girl sent to target a patrol of Sunni militiamen in the restive town of Baqouba, northeast of Baghdad.

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