SAMARRA (AFP) - US soldiers killed three unarmed Iraqi civilians, including a woman, near the central city of Samarra, the American military said Wednesday. The incident which is under official investigation occurred early Wednesday as US troops carried out what the military described as operations targeting Al-Qaeda in the central zone of the war-ravaged country. "The force engaged them, killing two men and a woman, and wounding another woman," the US statement said. But Nusayf Jassim, a resident on Samarra, 125km north of Baghdad, told AFP that US soldiers had raided his home in search of insurgents and in the ensuing gunfire his mother and two brothers were killed. Meanwhile, Iraq forces supported by US troops laid siege to the city of Baquba and arrested 35 suspects on Wednesday, the second day of a major assault on Al-Qaeda fighters in dangerous Diyala province. General Ali Gedan, director of military operations in Diyala, told AFP a tight security cordon had been thrown around the provincial capital Baquba, with scores of police checkpoints set up to prevent non-residents from entering or leaving. The capital was also under a 24-hour traffic curfew, hampering the movement of goods and people inside the sprawling city. "Up to midday our forces detained 35 wanted for terrorism and others involved in crime, murder and setting of explosions," defence ministry spokesman Muhammad al-Askari told AFP. "Iraqi security forces are controlling the entry routes into Diyala to stop terrorists from escaping to other provinces," Askari added. The offensive follows similar Iraqi military operations in the southern provinces of Basra and Maysan, and the northern province of Nineveh, as Iraq forces try to shore-up tenuous security gains in the war-ravaged country. Fed by the Euphrates and Diyala rivers, Diyala was once the granary of Iraq and its lush orchard made it the country's orange capital, but its multi-ethnic population has proven one of the most dangerous to control.