KABUL, May 3 (Xinhua) -- At least six civilians were killed and over 20 others injured after a suicide bomber struck a foreign forces' military convoy in a diplomatic district in central Kabul on Wednesday, an Afghan source said.

"The obvious target was a U.S. military convoy moving in Shashdarak locality where several diplomatic offices and government buildings are located. The explosion occurred roughly at 08:00 a.m. local time (0330 GMT) and details will be shared with media after an ongoing investigation," the source told Xinhua anonymously.

Government troops cordoned off the area shortly after the blast, keeping people from gathering at the scene for fears that there might be a second blast.

A public health official told local media that six civilians were killed and more than 20 others injured as the blast took place at morning rush hour.

A thick black smoke was seen rise above the site following the blast.

So far, no one has claimed responsibility for the attack.

On April 12, a suicide bomber targeted a vehicle of presidential palace security personnel, killing five people and injuring three others in neighboring Muradkhani district.