The U.N.-contracted chopper with nine passengers was conducting an air medical evacuation when a technical problem forced it to make an emergency landing near Hindhere village, an area controlled by the Islamist militant group.
The security sources asked to remain anonymous due to the sensitivity of the issue.
One security source and another person familiar with the matter said four Ukrainians were on the helicopter as crew members.
Somalia's government said on Thursday it was working to rescue hostages, but military officers warned that such attempts would be difficult in an area they said has been under the al Qaeda-affiliated group's control for more than a decade.
An internal U.N. memo seen by Reuters said one passenger had allegedly been killed, while six were taken hostage by al Shabaab militants. Two people fled and their whereabouts are not known, it said.
The Ukrainian government has not commented on the issue. Ugandan army representatives said they did not have any information. The Egyptian government could not immediately be reached for comment.
Separately, the United Nations Assistance Mission in Somalia (UNSOM) said on Friday that a U.N. guard was killed in a mortar attack by suspected al Shabaab militants near the capital Mogadishu's Aden Adde international airport.
The attack took place on Thursday night when several mortar rounds landed inside the airport area where the U.N. compound is located, UNSOM said.
Al Shabaab could not be reached for comment.
The militants, who control vast areas of the south and centre of the country, have been waging an insurgency against the Somali government since 2006 in an attempt to establish their own rule based on their interpretation of Islamic law.
(Reporting by Abdi Sheikh in Mogadishu and Giulia Paravicini in Nairobi, additional reporting by Tom Balmforth in London, Writing by Bhargav Acharya, Editing by Aaron Ross and Hugh Lawson)
By Giulia Paravicini
