Officials forewarned

Iraq's interior ministry said the authorities were forewarned of the attack but were unsure when it would happen.

"The truck exploded 50 metres before reaching its target (the base)," Major-General Abdul Karim Khalaf, a ministry spokesman, said.

"There was more than 1,000 kilogrammes of explosives in the truck, which levelled three buildings [near the base]."

Iraqi forces opened heavy fire on the lorry after it ignored a request to stop at a checkpoint on the approach to the base.

Mosul violence

Al-Qaeda and other fighters have proven hard to defeat in Mosul, even as the violence set off by the US-led invasion of 2003 has waned elsewhere in Iraq.   

Fighters have exploited the divisions among Mosul's patchwork of Kurds, Sunni Arabs, Christians and other groups to remain effective, and are also known to retreat to hideouts in the remote and mountainous region surrounding the city.

The number of US soldiers killed in action in March was the lowest since the invasion of Iraq in 2003.

In February four US soldiers were killed in a single attack.

At least 4,200 US troops and tens of thousands of Iraqis have been killed since the invasion.