BAGHDAD, June 8 (Reuters) - A vehicle driven by a suicide bomber blew up near a patrol base in Iraq on Sunday, killing one U.S. soldier and wounding 18 others, U.S. forces said.

Two Iraqi contractors were also wounded in the attack in Kirkuk province, about 250 km (155 miles) north of Baghdad, the U.S. military said in a statement.

The casualties were taken to a military hospital, it said.

Iraqi police and medical sources said five civilians were wounded in the attack, which they said was by a suicide truck bomb against a joint Iraqi and U.S. base at al-Rashad, 25 km (15 miles) southwest of the city of Kirkuk.

Three U.S. soldiers were killed by gunmen in the Sunni Arab town of Hawija, near Kirkuk, last Wednesday.

U.S. and Iraqi officials have highlighted an improvement in security in Iraq that has led to sharp drops in violence and in attacks on U.S. soldiers five years after the U.S.-led invasion to topple Saddam Hussein. But U.S. commanders have said that security improvements are fragile and reversible.

In May, the number of U.S. soldiers killed in Iraq fell to 19, the lowest monthly death toll in a five-year-old war that has claimed the lives of more than 4,000 American soldiers. (Reporting by Adrian Croft, Waleed Ibrahim; Editing by Janet Lawrence)

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