An American soldier killed five of his fellow troops at a base in Baghdad, the Pentagon said today.

The five were killed in a shooting 2pm local time, at Camp Liberty near Baghdad international airport.

An investigation into the circumstances surrounding the deaths is underway. The toll was the highest for US personnel in a single attack since April 10, when a suicide truck driver killed five American soldiers with a blast near a police headquarters in the northern city of Mosul.

In a separate incident, a US soldier was killed yesterday when a roadside bomb exploded near his vehicle in Basra province of southern Baghdad.

Earlier this month, two US soldiers were killed by a man wearing an Iraqi army uniform at an Iraqi military training centre in northern Iraq.

Violence has dropped sharply in Iraq, but insurgent attacks continue, and a rash of big bombings has raised questions about security less than two months before US forces are due to withdraw combat troops from urban bases.

Thirteen US soldiers were killed in combat in April, including the five who died in Mosul.

In other violence, a senior Iraqi traffic officer was assassinated on his way to work in Baghdad, in the second attack on a high-ranking traffic police officer in the capital in as many days.

A car cut off Brigadier General Abdul-Hussein al-Kadhoumi as he drove through a central square in the capital. Gunmen in a second vehicle pulled up alongside and opened fire, police said, citing witnesses. Al-Kadhoumi was director of operations for the traffic authority.

The gunmen were armed with pistols equipped with silencers, according to police. Incidents involving gunmen armed with sophisticated weapons, including silencers, have been on the rise since a string of high-profile robberies in April.

The attack came one day after the convoy of the Baghdad traffic department head, Major General Jaafar Toma, was hit by a roadside bomb. Four of his bodyguards were wounded but the general escaped unharmed.

Police today reported discovering the body of a 5-year-old Christian boy, Tony Edward Shawel, who had been kidnapped 10 days earlier in the town of Sheikhan, near Mosul. The body was found yesterday and police said it showed signs of torture. His family had been negotiating with the kidnappers to reduce the ransom from the originally demanded $50,000 (£33,127).