A Pakistani military official said on Saturday that gunmen opened fire near the army's headquarters outside the capital of Islamabad.
The assault in the morning sparked an intense shootout that was ongoing Saturday.
A senior army officer said troops were returning fire and top officers were trapped inside. The official spoke on condition of anonymity because he was not authorized to speak to the media.
The brazen attack on the army came a day after a bomb killed nearly 50 people in the northwestern city of Peshawar.
Pakistan has been fighting an ever-intensifying campaign against Islamist militants including the Taliban and groups linked to al-Qaida.
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