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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Former Chancellor Lord Geoffrey Howe was Margaret Thatcher's longest-serving Cabinet minister, also taking on the role of Foreign Secretary.
Lord Geoffrey Howe is known by many for his resignation speech which was seen as the catalyst which destroyed Margaret Thatcher.