Dennis Cassidy, 49, works for the European Union's Somalia Unit. He was kidnapped last week with two Kenyans, an Indian and a Canadian, all of whom worked for the UN.
Somaliland officials said that the workers were on a mission to halt the destruction of local trees, which are cut down and used to make charcoal.
It is suspected that they were kidnapped because they were thought to be carrying money to buy off the charcoal-makers.
The five were thought to have been taken at El Ayo, on the Gulf of Aden in the north-east corner of Somaliland, a region of Somalia which declared its independence from the south in 1991.
It was not known if they had been hurt. "The UN is not going to say anything because it is an ongoing hostage situation," a UN spokesman said.
The area has been relatively peaceful since declaring its independence. However, else- where in Somalia, aid workers frequently have been the target of the warring factions that have been vying for control of the country since a 1991 coup ousted presiden Siad Barre and left the country without any central government.Reuse content