The explosion, in the Shashdarak area close to buildings of the NDS intelligence service, came a week after a blast killed 60 people at a voter registration center in the west of the city.
A Kabul police spokesman confirmed the blast and said it might have been caused by a suicide attack, but there was no confirmation and no immediate claim of responsibility.
The Taliban, fighting to restore their version of strict Islamic law to Afghanistan, announced their usual spring offensive last week and there has been heavy fighting in several areas of the country since.
Security officials have warned of the risk of increasing attacks in Kabul and elsewhere ahead of parliamentary elections planned in October.
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