KABUL, June 30 -- Two explosions hit an Afghan police convoy in the western part of Kabul on Thursday, leaving more than 20 people dead, sources said.

"The initial investigation found a five-bus police convoy was targeted by two blasts in Qala-e-Haidar Khan locality in the morning, leaving casualties," a police officer told Xinhua anonymously.

More details would be made public later as investigation was underway to find more facts about the incident, the source added.

Unofficial sources said more than 20 people were killed in the attack.

The convoy was reportedly transporting hundreds of police officials and cadets from a police training center in neighboring Wardak province to central Kabul.

Security forces have cordoned off the area for precautionary measures.

A thick black smoke was seen rise above the site.

The Taliban militant group has claimed responsibility for the attack.

Zabiullah Mujahid, a purported Taliban spokesman, told local media that a Taliban suicide squad killed 27 police officials and injured several others in the attack.