South Sudan's president appealed for calm and pledged his country would "not go back to war," after a United Nations helicopter was attacked and a crew member killed during a Friday rescue mission.

A fragile power-sharing agreement between President Salva Kiir and First Vice President Riek Machar has been threatened in recent weeks by clashes between their allied forces in the northeastern Upper Nile State.

The U.N. Mission in South Sudan (UNMISS) said its team was attempting to extract members of the South Sudanese army from the area when its helicopter came under fire, killing a crew member and seriously injuring two others.

A South Sudanese army general and other officers were killed in the failed rescue mission, UNMISS said in a statement, saying the incident might constitute a war crime.

Kiir urged citizens to remain calm, stating: "I have said it time and again that our country will not go back to war. Let no one take law into their hands."

"The government which I lead will handle this crisis. We will remain steadfast in the path of peace," he added.

South Sudan, the world's youngest country, ended a five-year civil war in 2018 with a power-sharing agreement between rivals Kiir and Machar.

But Kiir's allies have accused Machar's forces of fomenting unrest in Nasir County, Upper Nile State, in league with the so-called White Army, a loose band of armed youths in the region from the same ethnic Nuer community as the vice president.

Late Friday, local media reported a statement from Machar's office that condemned the "barbaric act."

Efforts to "restore peace in the region remain a top priority," the statement added, with Machar "continuing to engage all stakeholders to prevent further violence."

"The attack on UNMISS personnel is utterly abhorrent and may constitute a war crime under international law," said head of UNMISS Nicholas Haysom.

"We also regret the killing of those that we were attempting to extract," he added.

U.N. secretary-general spokesperson Stephane Dujarric urged an investigation "to determine those responsible and hold them accountable."

A government garrison in the region was overrun by the rebels Tuesday, the information minister told reporters earlier this week, adding that a general and several soldiers had survived the attack and were still fighting the rebels.

Kiir's government responded with multiple arrests of Machar's allies in the capital, Juba, including Petroleum Minister Puot Kang Chol, deputy army chief General Gabriel Duop Lam and Peacebuilding Minister Stephen Par Kuol.

The latter was released Friday, according to his spokesperson.

UNMISS said its evacuation mission was an attempt to end the violence in Nasir County that had caused "significant casualties and civilian displacement."

Regional and Western diplomats warned earlier this week that the events threaten the 2018 peace agreement that ended a civil war that left 400,000 people dead.

"Juba-based leaders must demonstrate their commitment to peaceful dialogue and should put the interest of the South Sudanese people first," said a joint statement from a group of embassies that included the United States, Britain and the European Union.

UNMISS also called on the parties to "adhere to their commitment to uphold the ceasefire and protect the integrity" of the peace agreement.

There also has been criticism of recent political moves by Kiir, described by analysts as attempts to consolidate his position and sideline Machar.

Last month, Kiir fired two of the five vice presidents in his unity government without consulting other stakeholders, and he removed the governor of Western Equatoria State, a member of Machar's movement.

Fears of increased hostilities have grown.

"South Sudan is slipping rapidly toward full-blown war," said International Crisis Group Horn of Africa director Alan Boswell.

He urged the U.N. to prepare peacekeepers to save civilian lives, adding: "We fear large-scale ethnic massacres if the situation is not soon contained."

In Juba, the embassies of Canada, France, Germany, the Netherlands, Norway, the United Kingdom, the United States and the European Union delegation condemned the attack in a joint statement and urged dialogue "at the highest level to prevent further violence and loss of life."