"I was in the area when the blast went off and this minibus was blown to pieces," one eyewitness said.
Ethiopian authorities have not provided details on casualties, but witnesses have said that police have sealed off streets leading to the foreign ministry.
The explosion comes just over a month after three people were killed and 18 wounded in bomb blasts at petrol stations in Addis Ababa.
Ethiopian authorities blamed the April 14 attack on the Oromo Liberation Front (OLF), a separatist group.
The OLF, alongside the Ogaden National Liberation Front and Ethiopia's rival Eritrea, are routinely blamed for such attacks.