ISLAMABAD: A team of gunmen wielding assault rifles and grenades tried to break into Pakistan's army headquarters Saturday, sparking a ferocious 45-minute gunbattle outside the capital that ended when all the assailants were killed, authorities said.

The audacious assault, which killed and injured several troops, 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 operationsagainst their forces and the killing of Taliban leader Baitullah Mehsud in a CIA drone attack in August.

The four gunmen, dressed in camouflage uniforms, drove in a white van up to the army compound in the garrison city of Rawalpindi shortly before noon Saturday and tried to force their way inside before being stopped by soldiers, security officials said.

The heavily armed attackers jumped out of van, took up positions throughout the area and began firing at the troops, said Mohammed Jamil, a police official.

One gunman hurled a grenade, and others fired sporadically from their hiding places at those manning the checkpoint at the compound's entrance, said a senior military official inside. The official, who said top army officials were trapped in the compound during the assault, spoke on condition of anonymity because he was not authorized to speak to the media.

Police and troops backed by helicopter gunships cordoned off the area, which is adjacent to the capital ofIslamabad.

After a 45-minute gunfight, all four assailants were killed, army spokesman Maj. Gen. Athar Abbas said.

"The situation is now under full control," he said.

Several soldiers were killed and injured in the attack, but the full scope of casualties was not immediately clear, said an intelligence official who spoke on condition of anonymity because he was not authorized to speak to the media.