Militants shot down an Iraqi army helicopter on Thursday and killed nearly a dozen security forces in overnight clashes, a regional official said, in what appeared to be an al-Qaida surge to retake one of its former strongholds in Iraq.

The fighting around the town of Hadid, about eight miles north of Baquba, the capital of Diyala province, was in its third and deadliest day.

It comes after a warning last weekend by al-Qaida’s leader in Iraq to push back into areas it was forced out of by the US military. That threat was followed by a wave of violence that killed 115 people in the country’s worst daily death toll in more than two years – an assault for which al-Qaida claimed responsibility.

A spokesman for the province, Salih Ebressim Khalil, said militants opened fire at the Iraqi army helicopter, killing one soldier, wounding another and forcing it to make an emergency landing. The rest of the crew were unharmed.

Overnight clashes on the ground killed 11 federal policemen, Khalil said. Diyala is a predominantly Sunni province between Baghdad and the Iranian border. It has a large Shia population, and well as pockets of ethnic Kurds, and long has been a battleground for Sunni insurgents trying to assert control. Its remote rural areas have served as a safe haven for insurgents, and posed a challenge to Iraqi security forces.

In a statement posted on a militant site on Saturday, local al-Qaida leader Abu Bakr al-Baghdadi announced a new campaign he called “Breaking the Walls”. He said it sought to undermine Iraq’s Shia-led government by realigning with Sunni tribes, and return al-Qaida to areas it was driven from before the US military withdrawal in December.