South Sudan's government says it is sending more troops and police to the town of Pibor, which has seen an outbreak of ethnic violence.
On Saturday, members of the Lou Nuer group attacked Pibor, home to the rival Murle group, in the latest of a series of reprisal attacks over cattle raids.
Charity Medecins Sans Frontieres says it is "extremely worried" after losing contact with some 130 staff in Pibor.
A hospital and other parts of the town were reportedly set alight.
The number of casualties is not yet known.
The Lou Nuer fighters are reported to be moving south, chasing the Murle. South Sudan's President Salva Kiir has called on them to stop their advance.
The government said it was deploying more troops and an additional 2,000 police to Pibor.
Military spokesman Col Philip Aguer said: "The 2,000 police are being sent within the next 24 hours. Troops will be deployed as soon as possible."
The UN had sent more more peacekeepers to defend the town on Friday following reports that the armed Lou Nuer men were approaching.
But questions will be asked as to why hundreds of South Sudanese soldiers and UN troops were unable to protect Pibor, says the BBC's East Africa correspondent Will Ross.
A spokesman for Medecins Sans Frontieres (MSF) 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.
Parthesarathy Rajendran 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 Pibor amid fears of an impending assault.
Six thousand fighters from the Lou Nuer ethnic group have been marching through Jonglei state in recent weeks setting fire to homes and seizing livestock.
The entire town of Lukangol was burnt to the ground last week. About 20,000 civilians managed to flee before the attack, but dozens were killed on both sides.
About 1,000 people have been killed in Jonglei in recent months, during inter-ethnic fighting, triggered by the cattle raids.
The governor of Jonglei state and the vice-president of South Sudan have been trying to mediate between the rival ethnic groups.
Our correspondent says the Lou Nuer and Murle communities are now basically at war with each other.
Tribal violence over cattle rustling has been going on for years in the region, but reprisal attacks are now at an alarming level, our correspondent adds.
South Sudan became independent on 9 July 2011 following decades of civil war with the north.
One legacy of the conflict is that the region is still flooded with weapons.
These are now being used in tribal power-struggles, which often focus on cattle because of the central role they play in many South Sudanese communities.
So far, the South Sudanese authorities appear unable to make any progress in tackling the problem.