MOSUL, Iraq - Under a hail of gunfire, a suicide bomber charged a checkpoint in northern Iraq on Friday, detonating a truck laden with explosives and killing five U.S. troops and two Iraqi policemen.

The U.S. military gave the death toll in a statement and said the attack in the restive city of Mosul was the single deadliest incident for U.S. soldiers in Iraq in over a year.

Mosul, some 390 km (240 miles) north of Baghdad, is one of two regions of Iraq where U.S. forces are still locked in major combat operations against al Qaeda and other insurgents, despite a drop in violence elsewhere in Iraq over the past year.

Iraq's Interior Ministry said the authorities had been warned of such an attack but were unsure when it might happen.

U.S. and Iraqi forces opened heavy fire on the truck after it ignored a request to stop at a checkpoint close to an Iraqi police base in southwest Mosul.

The truck exploded 50 meters before reaching its target (the base), Interior Ministry spokesman Major General Abdul Karim Khalaf said, adding that only one Iraqi policeman was killed in the attack. He could not confirm the U.S. casualties.

There was more than 1,000 kg of explosives in the truck, which leveled three buildings (near the base), he added.

The explosion left a huge crater and damaged buildings over 100 meters away from the blast site, an Iraqi policeman said.

Two U.S. soldiers and 20 members of the Iraqi security forces were wounded in the blast, the U.S. military said. Iraqi police said the blast wounded 70 people and destroyed five Iraqi and two U.S. armored vehicles.

At least two people suspected of being involved in the attack were detained, and the incident is under investigation, the U.S. military said.

The attack in Mosul and a string of explosions over the past week in Baghdad have raised questions about the readiness of Iraqi security forces to take control after U.S. troops leave.

U.S. President Barack Obama has pledged to withdraw U.S. combat troops from Iraq by the end of August next year, and all U.S. troops are due to leave by the end of 2011 as part of a deal signed with Iraq by former U.S. President George W. Bush.

The number of U.S. soldiers killed in action in Iraq last month was the lowest since the United States invaded in 2003 to oust Saddam Hussein. In February, four U.S. soldiers were killed in a single attack.

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

Insurgent groups 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.