The authorities in South Sudan have said at least 169 people, including senior officials, were killed on Sunday when a group of unidentified men launched an assault on an area in the north of the country.

Describing the attack as carried out by dozens of armed youth, Ruweng Administrative Area's Information Minister James Monyluak Mijok alleged that they came from neighbouring Unity state and were linked to the Sudan People's Liberation Army in Opposition (SPLA-IO).

The SPLA-IO has denied any involvement in the attack, accusing the Unity state authorities of politicising the violence.

The UN has repeatedly warned that the country is at risk of sliding back into full-scale civil war.

''The dead include 90 children, women and elderly people, as well as 79 members of regional forces, including police,'' Mijok said.

Fifty others were wounded, and the majority of them had been taken to the neighbouring Abyei Administrative Area, where they were being treated, the official added.

Mijok told the BBC that the attackers entered Abiemnom county in Ruweng before dawn on Sunday, at around 04:30 local time (02:30 GMT), when people were still sleeping and "surprised them".

He said the government forces on the ground "were outnumbered... The assailants set fire to homes and markets during fighting that lasted between three and four hours." Several senior local officials were killed, including the county commissioner and executive director.

Mijok said government forces had since driven the attackers out and that authorities were now in full control.

He also alleged that officials in Unity state "must have had knowledge" of the plan to target Ruweng. The Unity state authorities have not responded to this accusation. It is not clear what may have triggered the attack.

The United Nations Mission in South Sudan (Unmiss) said about 1,000 civilians began seeking protection near its base in the affected area. The mission said it was alarmed by the surge in violence in the region over the past 48 hours.

"Such violence places civilians at grave risk and must stop immediately," said Anita Kiki Gbeho, officer in charge of Unmiss.

"The mission has enhanced its protective posture and is working with the government of South Sudan to support urgent efforts to restore calm and safeguard affected communities," she added.

Peacekeepers are providing emergency medical care to at least 23 people wounded in the clashes. The mission has called on all parties to cease hostilities immediately and engage in dialogue.

Local reports and the AFP news agency say victims of the attack were buried in a mass grave on Sunday due to the high number of casualties and ongoing security concerns.

A similar incident in Abiemnom county last year left more than 42 civilians dead.

Following clashes in another part of the country - Jonglei sate - the medical charity MSF said 26 of its staff were missing after weeks of escalating violence between government and opposition forces.

The charity has now suspended medical services in two parts of the state - Lankien and Pieri.

The NGO added that its facility in Lankien was hit by a government air strike on 3 February.

"Many of our staff were forced to flee the violence alongside their families. Several are now displaced, sheltering in remote areas with little access to food, water or basic services," the statement added.

South Sudan, the world's youngest country, has been beset by civil war, poverty and corruption since it was formed in 2011.

The UN has warned that an "all-out civil war" could return as a power-sharing deal struck in 2018 between President Salva Kiir and his long-time rival, Riek Machar, has unravelled over the past year.