(CNN) -- Two military helicopters on their way to deployment in Somalia are missing and one has crash-landed, a Ugandan military spokesman said Monday.

Four attack helicopters were scheduled to refuel in Garissa, Kenya, said Ugandan Col. Felix Kulayigye. One landed safely, he said, while Kenyan air force officials told him the five-man crew of another helicopter was safe after a crash landing near Mount Kenya.

The other two helicopters were missing, Kulayigye said.

"It is too early to speculate the reason for the incidents," Kulayigye said.

The helicopters were part of the first deployment of air support for the African Union Mission in Somalia in Mogadishu, where Ugandan, Burundian and other African Union forces are fighting Al-Shabaab, an al Qaeda-linked group.

Al-Shabaab, which has battled Somalia's weak transitional government since 2007, controls much of southern Somalia and is active around the capital city of Mogadishu. The U.S. listed it as a terror organization in 2008.

Al-Shabaab claimed responsibility for suicide bombings that killed more than 70 people in Uganda in 2010 and has threatened attacks against U.S., Kenyan and Burundian interests in the region.

The four helicopters left Nanyuki, in Kenya's Rift Valley, after originally being dispatched last week from Entebbe, Uganda.