Two suicide bombers killed 16 recruits for the Iraqi security forces and wounded 30 others north of Baghdad on Tuesday, the U.S. military said, just days after Iraq promised a major security crackdown in the region.

A police source said 20 people were killed and 55 wounded.

The bombers blew themselves up in a queue at a recruitment centre in Baquba, 65 km (40 miles) northeast of Baghdad.

The U.S. military said the recruits were signing up for the police force, although Iraqi security officials said they were joining the army.

The attack is the latest in a string of bombings in recent months in volatile Diyala province, of which Baquba is the capital.

Sunni Islamist al Qaeda has sought to stoke tensions in religiously and ethnically mixed Diyala. It has frequently targeted policemen and police recruits.

Iraqi security forces are poised to launch a major crackdown in Diyala, the Interior Ministry said on Sunday, the latest in a series of operations aimed at stabilising the country.

"What's happening here is the enemy knows the government and security forces are getting stronger," said Major Peggy Kageleiry, a spokeswoman for the U.S. military in northern Iraq.

The Interior Ministry has not given a date for the start of the Diyala crackdown.

U.S. forces have been conducting security operations in Diyala since the beginning of the year and would take part in the new Iraqi push, the military has said.

Al Qaeda has sought to regroup in Iraq's north after sustained military campaigns pushed the militants out of their former strongholds in Iraq's western Anbar province and Baghdad.

The Diyala crackdown will be the latest Iraqi-led offensive aimed at stamping government authority on areas once in the hands of Sunni Arab insurgents or Shi'ite militias.

U.S. and Iraqi officials say a campaign against al Qaeda in the northern city of Mosul and surrounding Nineveh province has helped reduce violence there. Other operations have targeted Shi'ite militias in the southern provinces of Basra and Maysan.

Overall attacks across Iraq were down 85 percent in June from a year ago, the Iraqi military said last week.