A British man is one of two United Nations workers who have been shot dead in Somalia, the Foreign Office has said.
They were killed inside Galkayo Airport after getting off a plane, local security official Mohamed Mire said.
The attacker was dressed in a police uniform, an airport official added.
UK Foreign Secretary William Hague said: “I condemn these brutal murders in the strongest terms”.
Witness Hassan Ahmed said: “One of them died inside the airport and the other one was rushed to hospital where he later died of the injuries.”
The men were working for the United Nations Office on Drugs and Crime (UNODC), the UN confirmed.
Militants al-Shabab, the group – linked to al-Qaeda – battling the Somali government, celebrated the attack but refused to confirm or deny responsibility.
“UN organisations are always the enemy of Islam,” it said.
Mr Hague said the men had been helping to “deliver a better future for Somalia” and that he wanted the Somali authorities to “urgently investigate these murders” to bring the perpetrators to justice.
UNODC executive director Yury Fedotov described it as a “cruel and senseless attack”.
Nicholas Kay, Special Representative of the UN Secretary-General for Somalia added: “Our UN colleagues were working in support of the Somali people’s aspiration for a peaceful and stable future.
“There can be no justification for such a callous attack.
“I call on the authorities to conduct a full investigation immediately and bring the perpetrators to justice without delay.”
Galkayo lies 356 miles (574km) north of capital Mogadishu on the border with the semi-autonomous state of Puntland.
The region has become synonymous with piracy in the seas around Somalia, particularly in the Gulf of Aden.
It is home to many Somalis displaced by violence in the south where some attempt to make the sea crossing to Yemen.