RAWALPINDI, Pakistan — Gunmen wearing military uniforms and wielding assault rifles and grenades attacked Pakistan's army headquarters today, sparking a ferocious gunbattle outside the capital that killed four of the assailants, two senior officers and four other soldiers, authorities said.

Two of the attackers managed to infiltrate the heavily fortified compound in the garrison city of Rawalpindi, and troops were trying to flush them out hours after the initial assault, the military said. An Associated Press reporter at the scene heard four gunshots from inside the compound, long after an army spokesman said the situation was under control.

The audacious assault was the third major militant attack in Pakistan in a week and came as the government said it was planning an imminent offensive against Islamist militants in their strongholds in the rugged mountains along the border with Afghanistan.

It showed that the militants retain the ability to strike at the very heart of Pakistan's security apparatus despite recent military operations against their forces and the killing of Taliban leader Baitullah Mehsud in a CIA drone attack in August.