Two foreign United Nations workers – a Briton and a Frenchman – have been shot dead on arrival at an airport in Galkayo, central Somalia, officials said.

A UN source confirmed the pair were international staff members with the UN Office on Drugs and Crime (UNODC).

A statement issued by the government of the northern breakaway state of Puntland in Somalia identified the two victims as Briton Simon Davis and Frenchman Clement Gorrissen.

Nicholas Kay, special representative of the UN secretary-general for Somalia, said: "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."

The UK Foreign Office said it was aware of the death of a British national in Somalia on Monday and was ready to provide consular assistance to the family. AFP reported that sources identified the second victim as French.

"Two white men have been shot inside the airport as they got off a plane," local security official Mohamed Mire said. An airport official said the attacker was dressed in a police uniform.

"One of them died inside the airport and the other one was rushed to hospital where he later died of the injuries. Both of them were white men," said Hassan Ahmed, who said he witnessed the incident.

Galkayo is 370 miles (575km) north of the capital Mogadishu and lies on the border with Puntland.

UN security in Galkayo, which is outside of effective central government control, is normally extremely tight.

The UNODC has been working to combat piracy in Puntland and has built a new prison in the state capital, Garowe, which opened last week to house pirates who have been sentenced in other countries in the region, notably the Seychelles. The UNODC said the building was a key part of its maritime crime programme in the Horn of Africa.

"The two were a Frenchman and a Briton and they were supposedly staying in Galkayo for two days before heading to Garowe," said Abdirisak Mohamed Dirir, general director of Puntland's anti-piracy department.

The two men had flown into Galkayo to meet Somali officials on the issue of regulating the money transfer services that replace a formal banking system in Somalia, and were looking at the financial flow of money related to Somalia's pirate attacks, according to reports.

The executive director of UNODC, Yury Fedotov, said: "I condemn in the strongest possible terms the killing of two of my colleagues in a cruel and senseless attack. I hope the relevant authorities in Somalia will undertake every effort to ensure that their killers are swiftly apprehended and brought to justice.

"I would also like to offer my most profound condolences to the family, loved ones, friends and colleagues of these two individuals, who were so committed and dedicated to UNODC's work."

The men were on a mission in Somalia to offer technical advice and work on "illicit money flows", the UN said. Davis and Gorrissen were described as irreplaceable.

Davis was said to have had a distinguished career in the Metropolitan police, specialising in tracking financial movements, before he joined the UNODC in 2012. He had worked on piracy. Gorrissen first worked for UNODC in 2010 and helped to develop its activities in the Horn of Africa.

The foreign secretary, William Hague, has called for an urgent investigation into the murders. He said: "I condemn these brutal murders in the strongest terms and offer my deep condolences to both families."

UN staff members have been regularly targeted in Somalia, where the fragile internationally backed government, supported by African Union troops, is battling al-Qaida-linked al Shabaab rebels, although the assaults have tended to be on a larger-scale.

In February, at least six people were killed in a suicide car bomb attack targeting a United Nations convoy close to Mogadishu's heavily fortified international airport, among them four local security escorts working for the UN.

In June last year, al-Shabaab carried out a raid against a UN compound in Mogadishu, killing 11 people.