Explosions and heavy weapons fire continued for a fifth day Monday, including attacks by government forces on the United Nations peacekeeping base and a camp housing some 28,000 thousand displaced civilians.
He said the compound was caught in a "crossfire".
Former South Sudan rebel leader Riek Machar has also called for a cease-fire, shortly after the country's president ordered the army to cease hostilities.
Up to 300 British soldiers from the Royal Engineers arrived in South Sudan last month, joining a United Nations peacekeeping force.
Kiir called for implementing the peace agreement between the government forces and previous opposition army led by Riek Machar. Government forces have reportedly been ordered back to their barracks. Some of them have set up businesses in Juba and others are working for various companies. "There is serious ongoing fighting between government and opposition forces, including near the airport, UNMISS locations, Jebel and elsewhere throughout Juba", the embassy posted on its Facebook page. There are now no flights arriving or leaving from the airport, he reports.
However, Uganda's military said it it meant to evacuate its nationals trapped in the conflict.
Last August, after intense pressure from the global community, Kiir and Machar signed a peace deal that called for a two-year transitional government of ministers and parliamentarians from the two sides before new elections.
"For peace to stand a chance, the UN Security Council must ensure that all countries halt any further flow of weapons into South Sudan by immediately imposing a comprehensive arms embargo".
"U.N. peacekeepers, they even run away", he said. A Chinese UN peacekeeper was killed and several Chinese and Rwandan peacekeepers injured, Japan' s UN ambassador, Koro Bessho, said on Sunday after the UN Security Council was briefed on the situation.
United Nations officials are very concerned the fighting could spread across the country.
"It rings through the whole city every time they fire", said the aid worker, who spoke on condition of anonymity because he was not authorized to speak to press.
Tens of thousands are estimated to have died in the two years of fighting, which also has displaced more than 2 million people.
South Sudan's civil war broke out in December 2013 after fighting between the Dinka and Nuer ethnic groups spread across the country.
The violence intensified on Monday; a Reuters witness saw two helicopters overhead firing in the direction of Machar's headquarters, while residents reported tanks on the street. However, spokesman for the Vice President said at least 150 were killed with scores of others wounded.