Unidentified fighters have shot down an Iraqi army helicopter in clashes that have killed at least 19 people including 12 policemen, a regional official has said.

The fighting around the town of Hadid on Thursday follows a warning last weekend from al-Qaeda's leader in Iraq to push back into areas the group was driven out of by the US military after sectarian fighting peaked in 2007. 

Diyala provincial spokesman Salih Ebressim Khalil said fighters opened fire on the helicopter, killing one soldier, wounding another and forcing the aircraft to make an emergency landing. The rest of the crew was unharmed.

The helicopter was called in to provide surveillance for security forces battling fighters since an attack late on Tuesday on a security checkpoint in a rural area near Hadid, about 70km northeast of Baghdad.

Despite police efforts to seal off the area, gunbattles raged overnight on Wednesday, resulting in the death of policemen and at least seven fighters. Eight others were arrested, officials said.

'Breaking the walls'

Diyala is a predominantly Sunni province that is sandwiched between Baghdad and the Iranian border.

It has a large Shia population, as well as pockets of ethnic Kurds, and long has been a battleground for Sunni fighters trying to assert control.

Its remote rural areas served as a safe haven for insurgents at the height of the nation's sectarian fighting between 2005 and 2007, and posed a major challenge to Iraqi security forces.

In a statement posted online last Saturday, local al-Qaeda leader Abu Bakr al-Baghdadi announced a new campaign dubbed "Breaking the Walls".

He said it sought to undermine the nation's weakened Shia-led government by realigning with Sunni tribes, and returning to areas it was driven from before the American military withdrew from Iraq last December.

Al-Qaeda in Iraq is regarded by Iraqi officials as significantly weaker than at the peak of its strength in 2006 and 2007, but it is still capable of spectacular mass-casualty attacks across the country.

A day after al-Baghdadi's threat, shootings and bombings killed 115 people in Iraq's deadliest day in more than two years - an assault for which the group claimed responsibility.