The aircraft encountered a defect shortly after taking off from Beledweyne city in central Somalia, Major Hassan Ali told Reuters, before it landed near Hindhere village, bordering Galguduud region.
"Two Somali men and several foreigners were onboard. It was also carrying medical supplies and it was supposed to transport injured soldiers from Galguduud region," he said.
The United Nations Assistance Mission in Somalia (UNSOM) later confirmed there had been an incident involving a UN-contracted helicopter that was conducting an air medical evaluation. It said it was gathering information about what happened and response efforts were under way.
A UN worker who wished to remain anonymous said earlier that the aircraft crew included five foreigners.
Two UN sources familiar with the matter told Reuters the aircraft carried nine passengers.
Reuters could not independently verify the identities of those captured or their nationalities.
Al Shabaab, linked to al Qaeda, has been waging an insurgency against the Somali government since 2006 in a bid to establish its own rule based on a strict interpretation of Islamic Sharia law.
While the government has managed to ward off the militants from several territories since the mid-2010s, al Shabaab controls swathes of land in southern and central Somalia and has continued to target civilians and stage attacks on military establishments.
Spokespeople for the Somalia government did not immediately respond to requests for comment.
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