South Sudan government forces have moved into Pibor in Jonglei state to tackle escalating tribal violence that has forced thousands of people to flee and put the United Nations on high alert.
The government and the UN were beefing up their forces as a column of 6,000 armed youths from the Lou Nuer tribe marched on the town pursuing the rival Murle tribe.
Barnaba Benjamin, South Sudan's minister of information, told Al Jazeera on Sunday that the government had taken significant steps to protect citizens.
"In the president's new year message yesterday, he already ordered the army to co-operate with the UN peacekeeping forces to make sure that civilian lives are protected," Benjamin said.
"Secondly, Pibor town is under the full control of the government and there are no casualties at the present moment."
The government has ordered nearly 1,500 troops into the area, he said.
"The government is firmly in charge of the situation and is taking very bold steps. They are deploying large numbers of infantry soldiers and police who are already moving towards Pibor," Lise Grande, the UN humanitarian co-ordinator for South Sudan, told the AFP news agency.
The UN has deployed peacekeeping reinforcements in Pibor over the past four days.
"We remain on high alert and are very concerned that civilians may be at risk," Grande said, adding that the UN was evacuating the most vulnerable civilians from the town and that the situation remained "very fluid".
"Our understanding is that there has been damage to the periphery of the town, including the torching of tukuls [thatched huts] and that the compound of one humanitarian organisation has been taken over.
"But it appears that in the main part of town damage is limited."
By Sunday evening, most of the tribal youths appeared to have entered the town, but had caused only limited damage to property, a humanitarian source in Juba, South Sudan's capital, told AFP news agency.
Earlier in the day, Doctors without Borders (Medecins sans Frontieres - MSF) said staff from its hospital in Pibor and its two outreach clinics in the area had for the most part fled into the bush.
"What we are hearing is that our clinic has been damaged and a lot of things looted," Parthesarathy Rajendran, head of the mission, told AFP.
"Since the start of the fighting one week before, the whole population has started displacing and running into the bush."
Members of the Murle tribe began fleeing towards Pibor several days ago after the Lou Nuer raided the town of Lukangol, which the MSF spokesman said had been reduced to ashes.
Grande said the World Food Programme flew in a helicopter into Pibor with food on Saturday after plans for an airlift on Friday were suspended following reports that security was deteriorating.
"As of yesterday there was no food in Pibor, so this was an urgent step," she said.
South Sudan's Vice-President Riek Machar has called on the Lou Nuer youth to pull back to their towns as per an agreement reached on December 28, which was meant to calm tensions.
The UN told Al Jazeera last week that its mission in South Sudan had a mandate to protect civilians
The deal has yet to be implemented, and spokesmen for the Lou Nuer were quoted as saying that they would not stop their advance on the Murle people until their rivals were disarmed.
A group calling itself the Nuer Youth White Army issued a statement on December 26 vowing to "wipe out the entire Murle tribe ... as the only solution to guarantee long-term security of Nuer cattle".
The group accuses the Murle of stealing Nuer cattle and killing members of their tribe since 2005, when a peace agreement ended two decades of bloody civil war and led to South Sudan's independence in 2011.
Neither the UN or South Sudan's former rebel army the SPLA have protected the Nuer, the group claimed.
"We the Nuer Youth have decided to fight the Murle, SPLA and the UN," it said.
About 1,000 people have been killed since June in violence that was initially triggered by raids to steal cattle.
The UN had raised the alarm in September, calling it a crisis that threatened to engulf the fledgling nation.
The raids involved "army-like" movements of people with new weapons and satellite phones, the UN said at the time.
Source: Al Jazeera and agencies