Several people have died in South Sudan after an attempt by the UN to evacuate members of the national army came under fire, the UN has said.

One crew member died when a UN helicopter was shot at, a statement from its mission in South Sudan, Unmiss, said. It added that an injured South Sudanese general and several other troops were also killed during the evacuation attempt in Upper Nile state.

South Sudan President Salva Kiir said a second UN helicopter managed to take off after the attack, only to crash land, killing all passengers on board.

Twenty-seven South Sudanese soldiers were killed in total, Michael Makuei, Kiir's information minister, is quoted by the Reuters news agency as saying.

The UN has not confirmed details about a second helicopter, but it did say the attack on the first aircraft "may constitute a war crime".

Weeks of fighting in Upper Nile has threatened an already fragile peace deal between President Kiir and Vice-President Riek Machar.

In 2013, a rift between the pair sparked a five-year civil war, during which 400,000 people were killed and 2.5 million forced from their homes.

A peace agreement was signed in 2018 but the situation has been fraught ever since.

The ongoing fighting in Upper Nile is between the armed forces and the White Army, an ethnic militia that was allied with Machar during the war.

In agreement with the White Army and South Sudan's army, Unmiss has been evacuating wounded troops from the conflict zone.

Unmiss head Nicholas Haysom said in a statement that the attack on his mission's personnel was "utterly abhorrent and may constitute a war crime under international law. We deeply regret the tragic loss of our colleague and express our sincere condolences to his loved ones.

"We also regret the killing of those that we were attempting to extract, particularly when assurances of safe passage had been received."

President Kiir said the army general who died in the attack was Gen Majur Dak, who led the forces stationed in Nasir, a region in Upper Nile.

Alongside the fighting, a slew of arrests has sparked concerns that South Sudan may see a return to war.

A number of Machar's allies, including the oil minister and a high-ranking army general, were detained earlier this week.

After Friday's attack, Kirr urged the nation to "remain calm".

"I have said time and again, that our country would not go back to war. Let no-one take the law into their own hands. The government, which I lead, will handle this crisis," he said.

South Sudan is the world's newest nation, after seceding from Sudan in 2011.