A large suicide bomb exploded near the defence ministry in central Kabul during a visit to the Afghan capital on Saturday by new US Defense Secretary Chuck Hagel, officials said.
A spokesman for the Afghan defence ministry said that there were civilian casualties but he did not give any further details.
"A suicide attacker has detonated himself within 30 metres (100 feet) of the defence ministry gate, there have been casualties among civilians, but defence ministry personnel have not suffered any casualties," Zahir Azimi told AFP.
One soldier covered in blood at the scene said he had helped carry five people from the attack site.
An official at the location told reporters that the bomber had arrived on a bicycle.
Gunfire erupted after the loud explosion was heard across the city, and reporters travelling with Hagel were informed that he was taken to a safe area as streets around the blast were closed off by security forces.
"We can confirm a suicide improvised explosive device attack, followed by small arms fire, near the Afghan ministry of defence compound in central Kabul this morning," a statement sent from the NATO-led military coalition said.
"No ISAF troops were directly involved."