US forces are facing an intense wave of attacks in Iraq

Police said the bomb blew up in front of a police patrol in the Ghazaliya district and at least three officers were among the dead.

Earlier, the US military said four American soldiers were killed in an attack on Tuesday evening near the northern oil refining town of Baiji.

Violence has intensified ahead of a deadline for drafting a constitution.

US Defence Secretary Donald Rumsfeld said the new constitution would be one of the most powerful weapons against insurgents.

A draft is due to be presented to the parliament by 15 August, but a BBC correspondent in Baghdad says there are no signs of a breakthrough yet.

Major attacks

The US military said five of its soldiers were wounded in the Ghazaliya blast.

Overnight, a US patrol was ambushed in the oil-producing town of Baiji, some 155 miles (250km) north of Baghdad.

Two US Humvee vehicles and a large armoured patrol car were burned out by the attack, the US military said.

There has been a string of major attacks and ambushes on US forces in the last three weeks, including the killing of 14 Marines in the town of Haditha.

At least 40 US personnel have lost their lives in the attacks.