LAHORE: Suspected suicide attackers detonated a car bomb Wednesday that destroyed a police building and sheared walls off a nearby office of Pakistan's top intelligence service in the eastern city Lahore. About 30 people were killed and at least 250 wounded.

Interior Minister Rehman Malik said the attack — one of the deadliest in Pakistan this year — could be retaliation for the government's military offensive to rout Taliban militants from the northwestern Swat Valley.

Recent assaults in Lahore, Pakistan's second-largest city sited near the Indian border, have heightened fears that militancy in nuclear-armed Pakistan is spreading well beyond the northwest region bordering Afghanistan.

No one immediately claimed responsibility for the bombing. Police said two suspects were detained.

Raja Riaz, a senior minister in the Punjab provincial government, told reporters about 30 people were killed. At least nine police and several intelligence agents were among the dead, officials said. The remainder of the dead and the bulk of the wounded were civilians caught in the midmorning blast in a busy downtown section of the city.

Sajjad Bhutta, another senior government official, told reporters more than 250 people were wounded.

A police call center was reduced to rubble in the blast, and walls at an nearby office of the Inter-Services Intelligence agency collapsed. The ceilings of several operating rooms in a nearby hospital fell in, injuring 20 people.

"The moment the blast happened, everything went dark in front of my eyes," witness Muhammad Ali said. "The way the blast happened, then gunfire, it looked as if there was a battle going on."

Sajjad Bhutta, a senior government official in Lahore, told reporters a car carrying several gunmen pulled up on a street between offices of the emergency police and the intelligence agency.

"As some people came out from that vehicle and started firing at the ISI office, the guards from inside that building returned fire," he said. As the shooting continued, the car exploded, he said.

Police had little chance to react to the gunshots before the blast.

"All of a sudden we heard a loud sound and the roof collapsed on us," said Mohammad Rehman, a police official who was wounded and taken to a nearby hospital. "First of all though, we heard the sound of gunfire, then the blast occurred."

Malik blamed the attack on militants that government forces are fighting in the northeastern Swat Valley and the border region where U.S. and other officials believe al-Qaida and Taliban militants are hiding and planning attacks against Western forces in Afghanistan.

"They are anti-state elements, and after being defeated in Swat they have moved to our big cities," Malik told the Express news channel.

U.S. officials and others in the past have accused the Inter-Services Intelligence agency of having links to militant groups, including the Taliban in Afghanistan. But the secretive agency has also been responsible for capturing and interrogating al-Qaida terrorist suspects and collecting intelligence that helps the military's campaign against militants in the border region.

President Asif Ali Zardari condemned the attack in a statement and said his government remained committed to rooting out terrorism.

U.S. Ambassador Anne W. Patterson expressed sympathy for the victims of Wednesday's attack and praise for the security forces for battling "our common foes."

"These attacks show the lengths extremist elements are willing to go to as they attempt to force their agenda onto a people who only wish to go about their daily lives in peace," she said in a statement.

The Swat offensive is seen as a test of the government's resolve against the spread of militancy, and is strongly backed by Washington. The army has said at least 1,100 militants have been killed in a month of fighting and that the Taliban is in retreat.

The military on Wednesday said troops had cleared militants out of Piochar, a village in a remote part of Swat that is the rear base for Taliban leader Maulana Fazlullah, and predicted that Mingora, the largest town in the valley, would be cleared of militants within three days.

Two other areas, Sultanwas and Mohmand, had also been emptied of militants and were now safe enough for refugees who have fled the fighting to return home.

It was the first time the military has invited some of the more than 2 million refugees from the region to return to their villages since the fighting began, setting off an exodus that aid officials have warned could turn into a humanitarian disaster.

In another part of the northwest, troops backed by helicopter gunships killed 10 suspected militants and captured a cache of weapons in raids on Siplapai town in South Waziristan, a Taliban stronghold, intelligence officials told The Associated Press on condition on anonymity because they were not authorized to reveal the information.

Wednesday's attack was the third in Lahore in recent months.

In March, a group of gunmen attacked Sri Lanka's visiting cricket team in the heart of the city, killing six police officers and a driver.

Later that month, gunmen raided a police academy on the city's outskirts, triggering an eight-hour standoff with security forces that left at least 12 dead.