An unknown number of aid workers are missing after air strikes hit a hospital in South Sudan, near the Ethiopian border, a charity group says.

The hospital, run by medical charity Doctors Without Borders (MSF) in Jonglei state, "was hit in an air strike by the government of South Sudan forces during the night on Tuesday", MSF said. The South Sudan government is yet to comment.

In a separate attack, MSF said its health facility in Pieri, also in Jonglei state, was on the same day looted by unknown assailants, leaving it "unusable for the local community".

"Our colleagues had to flee with the community and their fate and whereabouts are still unknown," the medical organisation said.

There has been a resurgence in fighting in Jonglei state recently between government forces and those loyal to First Vice-President Riek Machar, who has been suspended from his post after being accused of plotting to overthrow President Salva Kiir.

There are fears that the fighting could re-ignite a full-blown civil war in the world's youngest nation.

Jonglei, one of the most food-insecure parts of the country with severe health needs, has seen an estimated 280,000 people displaced by fighting and aerial bombardments since December, the UN said.

MSF said that ahead of Tuesday's first attack in the Lankien area, it had received information about a possible strike, so the hospital was "evacuated and patients were discharged hours before the attack".

"One staff member was injured, and the hospital's main warehouse was destroyed along with medical supplies," the group said in a statement.

Lankien has been at the centre of renewed fighting in recent weeks between government troops and opposition forces.

"The hospital's main warehouse was destroyed during the attack, and we lost most of our critical supplies for providing medical care," the group added.

It was not immediately clear who was behind the attack but MSF said: "The government of South Sudan armed forces are the only armed party with the capacity to perform aerial attacks in the country."

MSF said it was the only health provider serving around 250,000 people in Lankien and Pieri, warning that attacks on its facilities there "mean that local communities will be left without any healthcare".

"While we are aware of the enormous needs in the country, we find it unacceptable to be a target for attacks," said Gul Badshah, MSF's operations manager.

The medical charity said it had experienced eight targeted attacks in South Sudan last year, forcing the closure of two hospitals in Greater Upper Nile and the suspension of general healthcare activities in Jonglei, Upper Nile and Central Equatoria states.

In December, the South Sudanese government imposed restrictions on humanitarian access in opposition-held areas of Jonglei, limiting MSF's ability to deliver essential medical assistance.

Fighting between forces loyal to Machar and Kiir first broke out in 2013, two years after independence.

A 2018 peace deal ended the civil war that had killed nearly 400,000 people, but it has never been properly implemented and the relationship between the pair has become increasingly strained amid ethnic tensions and sporadic violence.

Machar is currently on trial on charges of murder, treason and crimes against humanity, which he denies.