The aid workers, who include two French women, one Bulgarian woman and a Belgian man, were taken captive near the border with Ethiopia.
The Reuters news agency reported that Farah Osman, who lives in the area, had said: "Heavily armed men with three battle-wagons and three small cars kidnapped the foreigners who landed a plane, and also some people waiting for them at the airstrip."
Somalia has been the scene of troubles ever since 1991 and now faces an Islamist and nationalist insurgency.
Foreign aid workers in the country face danger because they have become targets for the kidnappers there.
The current Somali government is internationally recognised, but has lost control of large parts of the country to an insurgency driven by Islamist and nationalist groups.
The government is backed by troops from neighbouring Ethiopia, who are deeply unpopular with many Somalis.
More than three million people in Somalia - almost half the population - are in acute need of food or medical aid, according to the UN.
