Ten people were killed when a suicide bomber detonated an explosives-laden car outside the United Nations Mine Action Service offices in the Somali capital, Mogadishu, a police official said.
Seven of the dead were UN guards.
Unlike previous attacks blamed on the al Qaida-linked militant group al-Shabab, gunmen did not accompany the suicide bomber, Captain Mohamed Hussein said.
He said the bomber tried to speed through the barrier at the UN office but guards opened fire on the car.
Capt Hussein said a second suicide blast targeted a checkpoint manned by Somali security forces near the African Union base in Mogadishu. Casualties there remain unclear.
Al-Shabab has claimed responsibility for the attacks, according to the group's Andalus radio station.
Al-Shabab is waging an insurgency against Somalia's weak UN-backed government.
Al-Shabab wants to establish an Islamic emirate in the country based on a strict form of Islam.
Eight soldiers were killed when an al-Shabab suicide car bomber targeted a Somalia military training camp and attackers then entered the base on foot earlier this month.