The diplomats believed the blast on Thursday had probably been caused by a bomb.
Citing reports from Mogadishu, the diplomats said the explosion appeared to have come from a device attached to a motorcycle that had been parked outside the former ministry of foreign affairs in an area of the ruined coastal city popularly known as K5.
Bomb explosions are relatively rare in Mogadishu but clan skirmishes are common.
There has been a series of assassinations of former soldiers and police officers in recent months by unidentified armed men. Visiting BBC producer Kate Peyton was shot dead in an unsolved attack in Mogadishu on 9 February.
A new Somali government still based in Kenya where it was formed at peace talks has asked the African Union and Arab League to supply 7500 peacekeepers to help disarm militiamen roaming the capital, though no decision on size has been made.
An AU team has been visiting Mogadishu in recent days to prepare for the deployment and it was due to fly out later on Thursday for another part of the country.
The Transitional Federal Government was formed last year in the safety of Nairobi, tasked with bringing order to a country torn apart by 14 years of strife between rival clan-based regional commanders.