BAGHDAD (Reuters) - A female suicide bomber killed nine people and wounded 12 when she blew herself up at a market in the northern Iraqi city of Baquba on Monday, police said.

There were no further details on the attack in the capital of Diyala province.

The use of female suicide bombers has become a popular tactic of al Qaeda to avoid detection by security forces.

Men make up most of the Iraqi security forces and strong cultural taboos prevent them searching women at checkpoints, which are set up at many markets in the country.

The U.S. military says there have been more than 20 female suicide bombings this year in Iraq. It has blamed the attacks on Sunni Islamist al Qaeda.

Many of the attacks have occurred in Diyala, an ethnically mixed region that was once overrun by al Qaeda.

Last month, a female suicide bomber blew herself up among policemen outside a restaurant in Baquba, killing 15 people.

Violence in Iraq is hovering at four-year lows but suicide bombers remain capable of carrying out large-scale attacks, the U.S. military has said.

A sustained military campaign against al Qaeda has pushed the group out of its traditional strongholds in Anbar province and Baghdad in the past year. But its fighters remain a threat in the north, especially in northern Nineveh province and its capital Mosul, as well as in Diyala.

(Writing by Tim Cocks, Editing by Dean Yates)