Three bombs hidden in motorcycles exploded in a southern Iraqi oil city Wednesday evening, killing seven people who were sitting at nearby cafes, Iraqi officials said.

Such attacks are rare in the city of Basra, a center of oil production 340 miles (550 kilometers) southeast of Baghdad.

In general, violence in Iraq has dropped considerably in recent years, but militants have been escalating their attacks as the U.S. prepares to withdraw its troops at the end of the year, showing that Iraq's government cannot control the country.

The bombs went off near three cafes all within about 500 feet (150 meters) of each other, where young people were sitting, drinking soft drinks and smoking water pipes, said Police Maj. Majid Hussein of the Basra police.

He said 24 people were wounded in the blasts.

A hospital official speaking on condition of anonymity confirmed the death toll. He did not want to be identified because he was not authorized to speak to reporters.

There was no immediate claim of responsibility for the attack. It was unclear whether the violence could have been the work of a Sunni militant group such as al-Qaida, which rarely operates in the overwhelmingly Shiite city of Basra.

Shiite militants have also been known to attack cafes or other places of entertainment as a way to enforce their conservative religious agenda.

"It is hard to control such terrorist acts, even if we put one policeman at each house gate," said Ahmed al-Sulaiti, the deputy head of Basra provincial council.

Many international oil companies doing business in southern Iraq have offices in Basra, and the U.S. has a consulate there as well, a reflection of the growing importance of Basra to Iraq's hopes of economic development.

In other incidents Wednesday, two people were killed when a roadside bomb went off in the northern city of Mosul, 225 miles (360 kilometers) northwest of Baghdad, local police and hospital officials said.

In Kirkuk, 180 miles (290 kilometers) north of Baghdad, a 12-year-old boy and his father were killed when a bomb attached to their vehicle exploded, said Police Brig. Gen. Sarhat Qadir.

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Associated Press writer Mazin Yahya contributed to this report.