CAIRO — Al Qaeda insurgents clashed with Iraqi security forces Thursday, the second attack this week in what Al Qaeda in Iraq’s leader has depicted as a new offensive aimed at recapturing lost ground.

At least 12 people were killed, including five Iraqi policemen and seven militants, according to a police source in Diyala Province. The Associated Press reported that an Iraqi helicopter was shot down in the attack, but Iraqi officials said it had only been damaged.

The fighting began Wednesday night in Hadid town, about 6 miles northwest of Baqubah, in Diyala Province. The area was formerly an insurgent stronghold.

On Sunday, the leader of Al Qaeda in Iraq, who goes by the pseudonym Abu Bakr al-Baghdadi, announced in an unusual audio recording, posted on a jihadi website, that an offensive was about to begin in which the insurgents would seek to regain ground they had held in Iraq before US forces helped oust them.

The next day, the group launched a coordinated series of at least 40 attacks that killed over 100 people in Iraq. Al Qaeda has also announced its intention to join the conflict in Syria on the side of the popular uprising there, although opposition leaders have disavowed any connection with extremist groups.