Two foreign workers with the United Nations in the central Somalia town of Galkayo were on Monday shot dead at the local airport by gunman wearing the local police uniform, police and witnesses said.

The two foreigners, whose nationalities were not yet known, working with UN Office on Drug and Crime (UNODC) were at the airport when the man in police uniform shot them, Mohamed Ali, a local police officer told Xinhua.

"The gunman is now in detention and is being investigated. One of the white men died on the spot while the other was rushed to hospital where he was preannounced dead," Ali said.

Witnesses said that the man shouted "Allahu Akbar", Arabic for Allah is great, when he started shooting at the victims. Local media reports say that the gunman is thought to be mentally unstable.

UN Somalia envoy Niclos Kay condemned the killing of the UNODC personnel as " brutal murders ". Kay said the two were "International consultants in Galkayo to support UNODC efforts in the region."

"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," Kay said in a statement.

The UN Somalia envoy called on local authorities to "conduct a full investigation immediately and bring the perpetrators to justice without delay". Kay reiterated United Nations commitment

to continuing its support to Somalia.

The town of Galkayo has been relatively peaceful and is under the control of two local administrations that run the north and south of the divided town.

The airport, where the incident occurred, locates in the northern part of the town controlled by the semi-autonomous region of Puntland.

The southern section of Galkayo is controlled by the Galmudug regional administration.

Al-Shabaab militants have carried out intermittent attacks in both sides of the city but the radical group has not managed to establish a base in Galkayo.