Gunmen detonated a car bomb near police and intelligence agency offices in Lahore on Wednesday, killing about 30 people and wounding more than 100 in one of Pakistan's deadliest attacks this year, officials said.

At least four men with rifles stepped from the car and opened fire on the intelligence agency building, then set off a massive blast when security guards returned fire, officials said.

Interior Minister Rehman Malik suggested the attack could be retaliation for the government's military offensive to rout Taliban militants from the northwestern Swat Valley.

Lahore is the country's second-largest city and sits near the Indian border, and assaults there have heightened fears that militancy in nuclear-armed Pakistan is spreading well beyond the northwest region bordering Afghanistan. Wednesday's attack was the third major strike in Lahore in recent months.

No one immediately claimed responsibility for the latest bombing. Police said one suspect was detained.

Raja Riaz, a senior minister in the Punjab provincial government, told reporters that about 30 people died. Fayyaz Ranjha, a senior health official, told state-run Pakistan Television that at least 116 people were wounded. Police put the number of injured at 250.

The explosion was so powerful it sheared the walls off buildings in a main business district. TV footage from the scene showed injured bystanders while emergency workers carried at least one person covered by a blanket to an ambulance.

Police official Mohammed Ashfaq said it was a suicide car bomber, and that the attack occurred outside the office of the emergency police service in Lahore. TV footage showed the nearby office of the Inter-Services Intelligence agency also was damaged.

Ashfaq said gunshots were heard immediately after the blast. An AP reporter saw dozens of troops entering the damaged building of the spy agency to supervise the rescue work, while gunshots were also heard from inside the building even one hour after the blast.

Earlier this year, a group of gunmen attacked Sri Lanka's visiting cricket team in the heart of Lahore, killing six police officers and a driver and wounding several of the players.

A few weeks later, gunmen raided a police academy on the city's outskirts, leaving at least 12 dead during an eight-hour standoff with security forces including army troops. Pakistani Taliban chief Baitullah Mehsud claimed responsibility for that attack.

The military is waging an offensive against Taliban militants in the northwest Swat Valley, a monthlong operation the army says has already left some 1,100 suspected militants dead. The offensive has spurred fears that the Taliban could stage revenge assaults.

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