JUBA, South Sudan — 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., when a large thunderstorm put a damper on the violence, U.N. mission spokeswoman Shantal Persaud said. 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, witnesses said.

“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 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.

The U.N. Security Council was holding a closed emergency meeting Sunday afternoon for consultations on 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,” Ban said 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.