ADDIS ABABA (AFP) - A helicopter en route to the United Nations mission in South Sudan crashed on Wednesday in Ethiopia, with four Russian crew members injured but no one killed, civil aviation officials said.

"An investigation is underway," Girma Gebre, accident investigation chief for Ethiopia's Civil Aviation Authority said on Thursday. "We are on the site now, we are looking for the cause of the incident." The helicopter crashed about 50km east of Addis Ababa's Bole airport.

Girma said no one had been killed but that the four crew members remained in hospital in Ethiopia.

"The helicopter is significantly damaged," Girma said.

The helicopter was flying to South Sudan from Djibouti and was scheduled to land in Addis Ababa to refuel before the crash took place.

Helicopters are key to delivering aid in South Sudan, a vast country with large expanses of swampland and few roads.