Pibor - Medical charity MSF says it is "extremely worried" after losing contact with some 130 staff in the town of Pibor in South Sudan.
An MSF spokesman said he believed they had fled into the bush after fighters from the Lou Nuer ethnic group attacked the town, home to the Murle group. A hospital was reportedly set alight.
UN troops and the South Sudanese army were unable to prevent the attack, which follows a wave of violence linked to cattle rustling.
MSF spokesman Parthesarathy Rajendran told the BBC they had only been able to get in touch with 13 members of staff, and believe the rest fled into the bush to escape the attack.
He urged both sides in the conflict to respect MSF facilities because the charity was the only health-care provider in the area.
Almost all the residents of Pibor had also already fled amid fears of an impending assault.
The UN had deployed more combat troops to defend the town following reports that the armed Lou Nuer men were approaching.
However, the 800 soldiers who were said to be present in the Pibor were completely outnumbered by the attackers.
The number of casualties is not yet known.