By Faisal Aziz

KARACHI (Reuters) - At least 15 people were killed and 100 injured in a suspected Taliban suicide car bomb attack on Thursday at a security compound in Pakistan's largest city Karachi where militants are held, officials said.

The attack took place at the compound of the police Crime Investigation Department, meters from the provincial chief minister's house in a central district known as the "red zone" because of its high security status.

Azam Tariq, a spokesman for the Tehrik-e-Taliban Pakistan (TTP), claimed responsibility for the attack which he said was in retaliation for U.S. air strikes in the country's northwestern tribal areas.

"It's a reaction to the drone strikes and such attacks will continue until drone strikes are stopped," he told Reuters,

The building is used to hold and interrogate a number of militants, including those from banned organizations. It was not immediately clear how many were inside at the time of the attack.

Sharmila Farooqi, a spokeswoman for the government of Sindh province where Karachi is the capital, said at least 15 people were killed and 100 injured.

"The attackers first opened fire and then rammed an explosive-laden vehicle into the building," senior police official Javed Akbar Riaz told Reuters. "We suspect that it was a suicide attack."

The blast left a crater about 40 feet across and 12 feet (four meters) deep in front on the building. The building was gutted and some parts of nearby buildings collapsed.

The U.S. Consulate, five-star hotels and other important buildings lie within a couple of kilometres of the blast site.

A Reuters photographer on the scene saw dozens of motorcycles destroyed and windows were shattered up to 2 km away. He also saw two wounded children evacuated from the scene.

"I heard shooting and then I came to the site and there were injured people screaming," said Sajid Khan, a bystander who was about one km away when the blast occurred. "Some women were screaming."

As the crowd looked on, rescue workers shouted Allah-u-Akbar (God is Great) as they removed slabs of cement and pulled out a body, its head covered with dust. Close by, a security guard knelt in the rubble and sifted through a pile of bullet casings.

(Additional reporting by Michael Georgy, Athar Hussain and Kamran Haider; Editing by Chris Allbritton/David Stamp)