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BAGHDAD - A suicide car bomber killed nine US soldiers at a military base north of Baghdad today, in one of the single deadliest insurgent attacks against American troops in the past year, the military said.

In a statement seen early on Tuesday, the military said 20 other US soldiers and one Iraqi civilian were wounded in the attack on the patrol base in volatile Diyala province.

The attack brings to 85 the number of US soldiers killed in Iraq this month, making April the deadliest since December, when 112 soldiers were killed.

The statement said 15 of the wounded soldiers were able to return to duty after medical care.

Tens of thousands of US and Iraqi troops have been deployed in Baghdad since February in a bid to curb rampant sectarian violence that has killed tens of thousands in the past year.

That has prompted insurgents to increasingly focus their attacks on provinces outside the capital.

A wave of up to 15 explosions resounded in Baghdad before dawn on Tuesday, apparently coming from the city's outskirts. A US military spokesman said he was not aware of any operation taking place at the time.

The blasts sounded like mortar shells or artillery.

While explosions often echo across the city, it is rare for so many to be heard in the early hours of the morning.

More than 3,300 US soldiers have been killed in Iraq since the 2003 invasion that toppled Saddam Hussein.