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By Dean Yates and Ibon Villelabeitia

BAGHDAD, April 24 (Reuters) - A suicide car bomber killed nine U.S. soldiers and wounded 20 others at a military outpost north of Baghdad on Monday in one of the worst attacks on American ground forces since the invasion in 2003.

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

In a statement seen early on Tuesday, the U.S. military said the bombing took place near Baquba, capital of Diyala province.

The volatile region has a mixed population of Sunni Arabs and Shi'ites and U.S. commanders sent extra troops there in March to combat entrenched insurgents and al Qaeda militants.

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

At least 85 U.S. troops have been killed in Iraq this month, making April the deadliest since December, when 112 were killed.

The military said 15 of the wounded soldiers were able to return to duty after medical treatment. One Iraqi civilian was also wounded.

Tens of thousands of U.S. and Iraqi troops have been deployed in Baghdad since February under the security crackdown that is seen as a last-ditch attempt to halt Iraq's slide into all-out sectarian civil war.

That has prompted insurgents to focus their attacks more on provinces outside the capital, where U.S. commanders say Sunni Arab insurgents and al Qaeda militants have regrouped.

In a separate attack on Monday, a suicide car bomber struck a gathering of senior police officials in Baquba, killing 10 policemen, including the city's police chief.

In February, a suicide attacker exploded a truck bomb outside a U.S. military outpost north of Baghdad, killing two U.S. soldiers and wounding 17.

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

U.S. President George W. Bush is under increasing pressure from Democrats to set a timetable for the withdrawal of the nearly 150,000 American troops in Iraq.

More than 3,300 U.S. soldiers have been killed since the 2003 invasion that toppled Saddam Hussein.

Bush's plan to send 30,000 additional troops has reduced the number of sectarian murders in the capital, but there has been a surge in car bombings inside and outside Baghdad.

U.S. Ambassador Ryan Crocker, in his first news conference since arriving in Baghdad, said on Monday he had been in discussions with the Iraqi government and U.S. officials on how to "take apart" the car bomb cells which have defied the two-month-old security clampdown in the capital.

A wave of up to 15 explosions resounded in Baghdad before dawn on Tuesday, apparently coming from the city's outskirts. A U.S. 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.

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