The explosion outside the Afghan defence ministry in Kabul has caused multiple casualties, Afghan officials have said.
A police spokesman said an apparent suicide attacker on a bicycle hit the main entrance to the building around 9:00am local time.
US defence officials have confirmed defence secretary Chuck Hagel, who is visiting the country, was in a safe location and unharmed.
He had been in a meeting at a coalition facility in the city at the time of the blast.
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