JUBA, South Sudan (AP) — South Sudan’s capital was rocked Sunday by heavy arms fire between forces loyal to the president and those of the vice president, causing widespread casualties and raising fears the country is returning to civil war.

The fighting began in the morning and continued until about 8 p.m. local time, when a large thunderstorm seemed to put a damper on the violence, said U.N. mission spokeswoman Shantal Persaud. She confirmed that a U.N. armored personnel carrier was hit by a shell at a camp to protect civilians. U.N. peacekeepers in the vehicle were wounded, said witnesses.

“The condition is really very bad. We have a lot of casualties this side, I think around 50 to 60 besides those of yesterday,” said Budbud Chol who oversees security at a clinic in the base. “We have civilian casualties. We have rocket-propelled grenades that have landed in the camp which has wounded eight people.” Among the wounded are five children and two women while the rest were men, he said.

At least one person has died in the camp, he said, but he did not know about casualties outside where the fighting was heavy.

The opposition side blamed government forces for starting the fighting Sunday morning with an attack on a rebel base in the Jebel area of the capital. Three helicopter gunships bombed rebel camps, said William Gatjiath Deng, a spokesman for the rebel forces.

South Sudan’s army confirmed the Sunday clashes but it is not clear how the fighting started, said army spokesman Lul Ruai Koang, who is in the SPLA general headquarters at Bilpham.

The U.N. Security Council will hold an emergency meeting Sunday to consider the fighting in South Sudan. U.N. Secretary General Ban Ki-moon condemned the fighting.

“I am shocked and appalled by the heavy fighting that is currently taking place in Juba. I strongly urge President Kiir and First Vice-President Riek Machar to do everything within their power to de-escalate the hostilities immediately and to order their respective forces to disengage and withdraw to their bases,” said Ban in a statement. “This senseless violence is unacceptable and has the potential of reversing the progress made so far in the peace process.”

Ban confirmed that U.N. compounds and sites to protect civilians in Juba have been caught in the cross-fire.

About 10,000 Juba residents fled neighborhoods where there was fighting, said Jeremiah Young, policy adviser for World Vision in South Sudan.

“We have seen quite a few individuals packing up and leaving, trying to find shelter, what look like a lot of civilians taking off down the street, carrying their suitcases, their children,” he said.

Other residents said they could not leave because of the fighting.

“I’ve gotten calls that I should leave but there was so much gunfire nearby I decided to stay in,” said one resident, who insisted on anonymity for her safety.

The fighting on Sunday appeared to be mainly in two areas: Jebel, where there is an opposition base and a U.N. base which houses thousands of internally displaced people, and in Gudele, where the rebels have another opposition base, including Machar’s compound. There were huge explosions in Gudele and people are fleeing by foot, said a resident, who spoke on condition of anonymity for fear for her safety.

“The situation in Juba has significantly deteriorated,” said a statement by the United States embassy. “There is serious fighting between government and opposition forces, including near the airport, U.N. mission locations, Jebel and elsewhere throughout Juba. U.S. citizens in Juba should remain vigilant … shelter in a safe location, preferably away from doors and windows, and avoid non-essential movements.”

Sunday’s fighting was a resumption of the conflict on Friday in which more than 100 people died. A precarious calm was restored on Saturday– the day South Sudan marked its fifth independence day — that was shattered Sunday by the fighting.

South Sudan is trying to emerge from a two-year civil war caused by political rivalry between Vice President Riek Machar and President Salva Kiir.

The two leaders issued a joint call for calm after Friday’s fighting which began outside the presidential compound where Kiir and Machar were meeting and soon spread through the city.

A similar skirmish in December 2013 sparked of the civil war that killed tens of thousands of people.

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