The latest violence in Juba comes two days after gun battles left at least 150 fighters dead. The fighting on Sunday saw the residence of South Sudan's vice president and former rebel leader Riek Machar. The massacre included the use of tanks and helicopter gunships. Regional airline Kenya Airways suspended flights to Juba, citing the "uncertain security situation", while the US embassy warned its citizens to stay indoors. The embassy said in a Facebook post: "The situation in Juba has significantly deteriorated.
"There is serious ongoing fighting between government and opposition forces, including near the airport, UNMISS locations, Jebel and elsewhere throughout Juba." Aid workers said civilians inside a UN refugee camp had been wounded in the crossfire. The violence comes a day after the world's youngest country marked its fifth independence anniversary.
It is a fresh blow to a peace deal that has failed to end the civil war that broke out in December 2013, after President Kiir accused Mr Machar of plotting a coup.
Fighting has continued despite the establishment of a unity government.
In a statement the UN Security Council said the violence showed a "lack of serious commitment" to peace on the part of both sides.
The civil war has claimed tens of thousands of lives and forced nearly three million people from their homes.
The humanitarian crisis has been accompanied by an economic one with the currency collapsing and inflation spiralling out of control.