LAHORE (Reuters) -- Pakistan's Taliban claimed responsibility for a suicide car bomb attack on a police intelligence unit in the city of Lahore that killed at least 13 people during morning rush hour today, officials said.
The Al-Qaeda-backed group threatened more violence unless U.S. drone aircraft strikes and Pakistani government offensives against its fighters stop.
Lahore's top administrator, Sajjad Bhutta, said up to 600 kilograms of explosives were used in the attack, which targeted a federal police office.
The violence may be a psychological setback for Pakistani authorities, who have won praise from Washington after capturing high-profile Afghan Taliban figures and made gains against homegrown militants in their ethnic Pashtun tribal bastions.
The blast was so powerful it wounded someone in a house about 300 meters away. Residents of the home stepped on a pool of blood as they cleared away a shattered doorway.
"Such attacks will continue as long as drone strikes and military operations go on in tribal areas," Taliban spokesman Azam Tariq told Reuters by telephone from an undisclosed location.
The blast left a huge crater in the road outside the office of the main police investigation agency, the Federal Investigation Agency, and destroyed the front of the building. The agency in Lahore has been attacked twice before.
Television showed footage familiar to Pakistanis who have grown tired of Taliban bombings of everything from girls' schools to crowded markets, as well as their government's inability to stop the bloodshed.
A man covered in blood was trapped in a car. Passers-by tried to help him out while rescuers searched through the rubble. A doctor at a hospital treating victims said the dead included a woman and a child.
Senior police official Ayaz Saleem said the death toll had climbed to 13, with 70 people wounded.
"There were three people in the car, but two of them had got out before the attack. One attacker died [in the blast]," he said.
Suicide bombings have eased in recent weeks but it is not clear whether that is because security has improved after military gains against the Pakistani Taliban or if the insurgents are merely regrouping for more attacks.
Nuclear-armed Pakistan has launched two big offensives in the northwest over the past year against the militants, who want to impose their austere version of Islamic rule.
The operations have destroyed militant bases, while Taliban leader Hakimullah Mehsud is widely believed to have been killed in a U.S. drone strike in January. But the Taliban have a history of bouncing back.
Some angry residents shouted at police as they arrived at the scene in Lahore's Model Town residential neighborhood.
"We repeatedly asked them to please move this office away from our houses, but they didn't give a damn," one woman said.
Stock market investors shrugged off the latest violence, having grown used to bomb attacks across the country, dealers said.
"The market has become sort of immune to these acts of terror and it only reacts if the damage is huge," said Khalid Iqbal Siddiqui, director at brokers Invest and Finance Securities.
Dealers said healthy foreign flows into the market in recent days had helped investor sentiment.
The Karachi Stock Exchange's benchmark 100-share index rose 1.2 percent today as Asian markets rose broadly.