The two UN workers shot dead in central Somalia were working on the links between money transfer systems and piracy, the UN anti-drugs and crime agency says.

Former British police officer Simon Davis, 57, and his French colleague, 28-year-old researcher Clement Gorrissen, were fatally shot by a man in uniform just after their arrival at the airport in Galkayo on Monday.

"The two men, who often worked together, were on mission in Somalia to offer technical advice and to help build local capacities in the specialised field of illicit money flows," the UN Office on Drugs and Crime said on Wednesday.

UNODC gave no details of the circumstances of the attack, nor on its motive, which remains unknown.

It said the two experts had been working to ensure that the remittance networks, which replace banks in Somalia, could be used by ordinary citizens but not by criminals. Many Somalis rely on remittance companies to receive money from relatives in the diaspora.

Davis served with the Metropolitan police force, specialising in tracking financial movements, before joining UNODC in 2012. He also worked closely with the British government in the area of fighting piracy.

In addition, Davis provided training to law enforcement officials in the Horn of Africa, UNODC said.

Gorrissen first worked for UNODC in 2010 as part of the Global Program against Money Laundering, Proceeds of Crime and the Financing of Terrorism.

In May 2011 he researched illicit money flows for the Contact Group on Piracy. He was a key contributor to the highly regarded report "Pirate Trails: Tracking the Illicit Financial Flows from Pirate Activities off the Horn of Africa".

No one has claimed the attack in Galkayo, a town that straddles the self-proclaimed autonomous regions of Galmudug and Puntland. Al-Shabab Islamists, who have been influential in the area around Galkayo, expressed their satisfaction at the killings but denied being behind them.

The 2013 report on financial transfers and piracy estimated that ransom payments totalling between $US339 million and $US413 million ($A363.29 million and $A442.59 million) were made to pirates between April 2005 and December 2012, with between 30 and 50 per cent of the total remaining in the hands of pirate chiefs.

Clan militia, pirate networks and criminal gangs control large chunks of Somalia, which has not had an effective central government since 1991.