The number of people killed in an attack in South Sudan on Sunday, when a group of unidentified men launched an assault in the north of the country, has risen to 178, a local official has told the BBC.

Describing the attack as carried out by dozens of armed youths, 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 authorities of politicising the violence.

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

Medical aid groups have described the scale of casualties as alarming, even for a region that has experienced repeated outbreaks of violence.

A diplomatic source quoted anonymously by the AFP news agency on Monday put the death toll at 70 but said it could increase.

''The dead include 90 children, women and elderly people, as well as 79 members of regional forces, including police,'' Mijok said, adding that many of them had been buried in a mass grave.

He said 73 people were still in hospital - many of whom were taken to neighbouring Abyei Administrative Area for treatment.

Medical charity Médecins Sans Frontières (MSF) described the situation in Abyei as extremely challenging.

"It was really difficult to manage the mass casualty situation, but many MSF and Ministry of Health staff came on their day off and from other facilities to support," Abraham Deng Lual Wek, an MSF nurse supervisor, told the BBC on Tuesday.

"For the surgical team, it was a challenge to manage all of the emergency cases between the two operating theatres," he said.

"Our triage area, emergency room and wards were full of patients, so we expanded capacity by using tents and a meeting space, which also filled quickly."

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.

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

Following clashes in another part of the country - Jonglei sate - 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.

Its facility in Lankien was hit by a government air strike on 3 February, it added.

"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 said.

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.