Heavy fighting erupted Sunday in the South Sudanese capital Juba, sending thousands of people fleeing following several days of clashes between government and rebel forces.
An aid worker who spoke on condition of anonymity estimated that about 10,000 people had fled.
Gregor Fischer, a German photographer, said there was "a lot of shooting" during the morning.
"We are hearing small gunfire and artillery." He also reported seeing government tanks and hearing helicopters.
Witnesses said much of the fighting took place in Jebel neighbourhood, where Vice President Riek Machar – rebel leader until a unity government was formed in April – has his residence.
Machar's spokesman James Gatdet said President Salva Kiir's forces had attacked Machar's positions in Jebel.
"Our forces have captured three tanks ... from President Kiir's forces who attacked Jebel site. They are repulsed. Their helicopter gunships have now stopped bombing Jebel after one of them was almost brought down," Gatdet said on Facebook.
Fighting then stopped, but the neighbourhood was still tense, Gatdet added. "Enemy forces may come back to attack our forces. There are still skirmishes in pockets of the town," he said.
Artillery shells reportedly landed in a UN base near Jebel, with "many casualties" including UN staff and displaced people, local broadcaster Radio Tamazuj said.
Officials at the president's office put the death toll at 270 people since Friday, including 210 casualties on Machar's side and 60 on Kiir's side.
Civilians were seen fleeing to two UN sites, according to Radio Tamazuj. Witnesses said thousands were also fleeing Juba to Gurei about 20 kilometres west of the capital.
The US embassy earlier reported fighting "throughout Juba," including the airport. Kenya Airways announced it had suspended flights to Juba "due to uncertain security situation."
The violence has been going on since Thursday, when clashes erupted between government and rebel troops.
On Friday, fighting broke out near the presidential palace, where Kiir was meeting Machar.
The outbreak of violence dealt a blow to hopes of peace after the warring parties had formed a transitional unity government in April, and just as South Sudan was marking the fifth anniversary of its independence from Sudan on Saturday.
The power struggle between Kiir and Machar, which escalated into a military confrontation in December 2013, has killed tens of thousands and displaced more than 2 million people.
The UN Security Council "strongly condemned" the fighting. The east African bloc Intergovernmental Authority on Development (IGAD), which mediated in the peace talks, said the violence had "once again placed the long-suffering people of South Sudan in unspeakable harm's way."