A “massive explosion” hit shortly after 9am followed by further blasts in the Tomping area of Juba, home to embassies, the airport and a UN base, said an aid worker.
“It rings through the whole city every time they fire,” said the aid worker, who spoke on condition of anonymity.
“I think one of the tanks must be near me, my ears are burning.”
Explosions and “very heavy gunfire” sounding “like popcorn,” was reported by a resident in the Gudele area.
Much fighting has centred around the UN base in the Jebel area, where 30,000 civilians have taken refuge.
The opposition also has a base near Jebel and their leader has his home there.
Two government helicopters have been bombing areas near the base while ground forces shell the base, including a camp of tens of thousands of displaced civilians, according to a source within the UN compound.
The displaced civilians are mostly of the Nuer ethnicity and sought protection from the UN after government-led killings of Nuer in Juba in 2013 which sparked the civil war.
UN peacekeepers have not fired at the troops shelling the base, said the source in the base, who accused the soldiers with UN blue helmets of abandoning their positions.
“UN peacekeepers, they even run away,” he said.
“They are not stopping it.”
UN peacekeepers in South Sudan are mandated to use lethal force to protect civilians under imminent threat in South Sudan.
Two UN peacekeepers from China were killed at the base on Sunday night, according to Chinese state media.
An eyewitness in the UN base said he saw a government tank fire on a Chinese armoured personnel carrier.
Video broadcast on Chinese state TV showed smoke rising after the attack, and Chinese peacekeepers tending to their wounded.
There were 67 injuries and 8 deaths in the UN base on Sunday, according to an internal situation report circulated among humanitarian organisations.
The fighting in the capital began last Thursday and continued through the weekend, when South Sudan marked the fifth anniversary of its independence from Sudan.
The gun battles in the capital are similar to fighting in December 2013 that sparked a two-year civil war in which tens of thousands died and displaced more than two million people.
President Salva Kiir and former rebel leader Riek Machar, who is now vice president, signed a peace accord last year and formed a transitional coalition government.
But fighting continued and the current clashesthreaten to plunge relatively stable parts of the country back into violence.