A bomb and gun attack on the offices of the Pakistani intelligence agency in the southern town of Sukkur on Wednesday killed four people and wounded at least 40, police officials said, in a brazen challenge to the country's powerful military.
More than an hour after the initial explosions, the gun battle was still raging as security services hunted up to 10 militants who detonated four bombs in the heavily guarded compound, Masood Bangash, the deputy superintendent of police in Sukkur, said.
The city headquarters of the Inter-Services Intelligence, the Pakistani military intelligence force, seemed to be the main target, police Deputy Inspector General Javeed Odho said.
It was unclear who carried out the attack, but previous such high-profile operations have always been claimed by the Taliban.
The intensity of the attack and the high-profile target unsettled a country that has long been inured to weekly bombings.
It also raises questions about the new government's aim of starting talks with militant groups.
Prime Minister Nawaz Sharif pledged to seek negotiations and reconciliation with militant groups who were willing to talk before he won last May's elections. But attacks have increased since he took office and the government has yet to present a security strategy.
Militants have launched such sophisticated attacks before. Last year, they attacked Kamra, a major airbase, and damaged an aircraft.
The year before, Taliban gunmen attacked a naval base in Pakistan's biggest city, Karachi. Ten military personnel were killed in the 16-hour assault.
In 2009, they attacked the national army headquarters in Rawalpindi, a city that is the stronghold of the armed forces.
Such attacks are embarrassing for Pakistan's military, which has ruled the country for more than half of its 65-year history.