A group linked to Al-Qaida in Iraq claimed responsibility yesterday for a suicide car bomb that killed nine US soldiers and wounded 20 in one of the worst attacks on US ground forces since the invasion in 2003.

"Two knights from the Islamic State in Iraq... driving two booby-trapped trucks hit the heart of the Crusader American headquarters in the region of Diyala," a statement from the Sunni group of the Islamic State in Iraq said in a Web posting.

The US military said only one suicide attacker was involved in Monday's attack on a military outpost at Diyala, north of Baghdad.

While frontal assaults by insurgents against heavily fortified US bases in Iraq are rare, a two-month-old security plan that places troops in less protected garrisons in Baghdad and neighboring areas has exposed them to greater risk.

The bombing came as President George W. Bush faced pressure from Democrats to set a timetable for the withdrawal of nearly 150,000 troops. Congress will vote this week on a funding bill that sets March 31, 2008 as the goal for pulling out most troops.

A US military statement said the attack took place near Baquba, capital of Diyala province, a religiously mixed area where US commanders last month sent 1,000 extra troops to combat entrenched Sunni Arab insurgents and Al-Qaida militants.

"We have seen a lot of recent attacks up in Diyala... that have been part of the fight for the province," said US military spokesman, Lieutenant Colonel Christopher Garver.

It was the second assault on such an outpost outside Baghdad in two months after a suicide truck bomber killed two soldiers and wounded 17 in February in what appeared to be a tactical shift by insurgents avoiding large US firepower.

At least 86 US troops have been killed in Iraq this month, making April the deadliest since December, when 112 were killed.

At least 3,333 US soldiers have been killed since the 2003 invasion that toppled Saddam Hussein.

The military said 15 of the wounded troops in Diyala returned to duty after medical treatment. One Iraqi civilian was also wounded.

In the previous worst ground attack against US forces in Iraq, 10 US Marines were killed near Falluja in a bombing on December 1, 2005.

Source: China Daily/agencies