The Somali armed Islamist group al-Shabab has seized a United Nations helicopter, along with about eight people, both passengers and crew, local sources have told the BBC.

The helicopter landed in territory held by the group in central Somalia.

The UN's mission in Somalia (Unsom) confirmed an "aviation incident" involving a UN-contracted helicopter conducting a medical evacuation.

It did not mention al-Shabab, but said "response efforts are under way".

A UN memo on the incident seen by AFP news agency says the helicopter crash-landed about 70km (43 miles) southeast of Dhusamareb and no UN staff were on board. The personnel were third-party contractors, the report said.

The nationalities of those on board have not been specified, nor the exact number involved. One passenger was reportedly killed and two others fled.

Al-Shabab controls large parts of southern and central Somalia.

The group is affiliated to al-Qaeda and has waged a brutal insurgency for nearly 20 years.

The seizure of the helicopter was confirmed to the BBC by Galmudug region Security Minister Mohamed Abdi Adan.

Several foreigners and two locals were on the helicopter, Somali military official Major Hassan Ali told Reuters news agency.

"It was also carrying medical supplies and it was supposed to transport injured soldiers from Galgudud region," he was quoted as saying.

The helicopter was heading to Wisil town near the frontlines of an offensive against al-Shabab by government forces when it landed.

The UN's World Food Programme (WFP) posted on social media on Wednesday night that the aircraft was "not a WFP or UN Humanitarian Air Service craft and no WFP personnel were aboard".

The WFP added that as a precaution, its flights in the area had been temporarily suspended.

The Somali government has in recent months intensified its fight against the al-Qaeda-linked group.