A UN helicopter crash-landed in an area controlled by armed Islamists in Somalia after its main rotor blade was hit by an object, a UN source has said.

Al-Shabab fighters seized the helicopter, with unverified reports suggesting that a passenger was shot dead, the source told the BBC.

A further six people on board are being held by the militants, while two reportedly escaped, the source added.

The helicopter was on a medical mission when it landed near a village.

The UN mission in Somalia confirmed an "aviation incident" on Wednesday involving a UN-contracted helicopter.

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

The UN source told the BBC one of the people on board was Somali, while the other eight were from elsewhere in Africa, and Europe.

The foreign nationals include the person reported to have been killed and the two who managed to escape. Their fate is unclear.

All nine, including the four-member crew, were third-party contractors, and not UN staff. Some of them were medics, the UN source said.

The helicopter was heading to Wisil town near the frontlines of a government offensive against al-Shabab when it crash-landed after being struck by an unspecified object, the source added.

Somali military official Major Hassan Ali told Reuters news agency on Wednesday that the aircraft was "carrying medical supplies and it was supposed to transport injured soldiers from Galgudud region".

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 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.