Four Ukrainians were on a United Nations helicopter seized by al-Shabab militants in central Somalia this week, Ukraine's foreign ministry said on Friday.

The U.N.-contracted chopper with nine aboard was conducting a medical evacuation when a technical problem forced it to land near Hindhere village, an area controlled by the Islamist group.

"Our citizens were members of the helicopter crew of the UN Mission in Somalia. ... Their identities have been established," Ukrainian spokesman Oleh Nikolenko wrote on Facebook.

He said that the aircraft belonged to a private Ukrainian company contracted to the United Nations, and that the government was contacting it to coordinate actions.

Security sources earlier told Reuters that nationals from Egypt, Uganda and Somalia were also on board. The sources asked to remain anonymous due to the sensitivity of the issue.

Somalia's government said on Thursday it was working to rescue the hostages, but military officers said it would be difficult in an area that has been under the al-Qaida-affiliated group's control for more than a decade.

An internal U.N. memo seen by Reuters said one person on the helicopter had allegedly been killed and six taken hostage. Two people fled and their whereabouts were not known, it said.

All U.N. flights in the area were suspended until further notice, the memo said.

Ugandan army representatives said they had no information. The Egyptian government could not be reached for comment.

Separately, the United Nations Assistance Mission in Somalia said a U.N. guard had been killed in a mortar attack by suspected al-Shabab militants near the capital's Aden Adde International Airport.

Mortar rounds landed on Thursday night inside the airport area where the U.N. compound is located, UNSOM said.

Al-Shabab could not be reached for comment.

The militants, who control vast areas of the south and center of Somalia, have been fighting the government since 2006 in an attempt to establish their own rule based on their interpretation of Islamic law.