Three teenage girls were killed in Iraq on Friday, two of them sisters, when one of them found a tape recorder wired with explosives and brought it inside their house, security officials said.

The incident took place in the town of Balad Ruz in volatile Diyala province, an ethnically and religiously mixed region northeast of Baghdad that has seen scores of attacks in recent years.

The two sisters who were killed were 14 and 16 years old. Their sister-in-law, who was 17, also died in the blast, said Ahmed Fuad, a medic at Baquba General Hospital in the provincial capital.

Another two girls, aged seven and 19, were wounded in the blast, Fuad said, adding that they were sisters of the two sisters who were killed.

The girls came from a Shiite family that lived in a predominantly Sunni part of the town. The family had recently returned after fleeing their home at the height of Iraq's sectarian fighting, security officials said.

The overall level of violence in Iraq has fallen sharply over the past year as US and Iraqi forces allied with local militias have flooded into previously ungovernable areas, but attacks are still common in certain areas.