(CNN) South Sudanese President Salva Kiir on Monday demanded an immediate end to the fighting between his soldiers and those loyal to his rival vice president. The civil disputes have left more than 150 dead across the capital city of Juba since fighting broke out on Thursday.

Following an overnight lull, fighting resumed Monday morning with the sound of gunfire blasting through parts of Juba, according to Shantal Persaud, acting spokeswoman for the U.N. Mission in South Sudan. The fighting threatens to throw the newest formed nation in the world back into a civil war.

The U.N. Security Council expressed its readiness to provide more security on the ground, and is calling upon nearby nations to prepare troops for possible deployment, said U.S. Ambassador to the United Nations Samantha Power.

Fighting first broke out Thursday with skirmishes between the Sudan People's Liberation Army, loyal to Kiir, and soldiers backing Vice President Riek Machar. The resurgence came after a halt the day before when the country celebrated the fifth anniversary of its independence from Sudan.

"What we may be seeing is a total breakdown of command and control in Juba," said Kate Almquist Knopf, director of the Africa Center for Strategic Studies. "We need to watch carefully for whether a cycle of reprisal killings by either side begins in the next few days."

The United States and India are evacuating nonemergency staff from their missions in South Sudan as bloody violence in the capital spirals, leaving scores of people dead, including two Chinese U.N. peacekeepers.

The International Committee of the Red Cross said it ceased almost all its operations in Juba on Friday due to fighting.

Persaud, the U.N. mission spokeswoman, talked to CNN by phone from a U.N. compound, taking cover in the bathroom as gunfire was exchanged outside the complex near Jubal's airport. Shots were also heard outside a U.N. civilian protection facility in the city's southeast.

Why they're fighting

Attack helicopters now engaging, to immediate north of UN House #Juba #SouthSudan pic.twitter.com/KZGrUGyGji — Paul Cruickshank (@pmcruickshank) July 11, 2016

The Security Council, which held a closed door meeting Sunday, expressed "shock and outrage" at attacks on civilians and U.N. compounds, saying they may constitute war crimes.

It called on Kiir and Machar to control their respective warring forces, prevent the spread of violence and commit themselves to implementing a ceasefire and peace agreement.

Machar said that soldiers on his side had been bombarded from helicopters.

In the last 2 hrs, we went through heavy bombardments by Pres Kiir helicopters. This tells that our partner is not interested in peace. 2/3 — Dr Riek Machar Teny (@Drriekmachar) 11 July 2016

An estimated 83,000 people are seeking refuge in churches, schools and outside the peacekeeping base of the U.N. mission, according to the United Nations.

Humanitarian workers have been stopped and threatened at checkpoints when trying to deliver assistance, unable to reach the most needy, according to Chaloka Beyani, the U.N. special rapporteur on the human rights of internally displaced persons.

International law requires that parties in conflict allow humanitarian workers to have safe passage, he said. The threats to humanitarians in the western city of Wau are unconscionable, his office said in a statement Monday, stressing that leaders of armed groups and forces must tell their troops to back off. Two weeks ago, fighting in Wau displaced at least 70,000, according to the United Nations.

"I am deeply disturbed by the renewed outbreak of violence in South Sudan, a country which marked its fifth anniversary of independence on Saturday with gunfire instead of celebration," Beyani said.

Gunshots, heavily armed exchange UN House area once again; going on now since approx. 08:25 @unmissmedia — UNMISS (@unmissmedia) July 10, 2016

"Everything came to a halt because there was too much confusion, too much shooting, too much commotion in town. Checkpoints are making movements impossible," said Jurg Eglin, head of the Red Cross mission in the country.

Stranded, hungry and scared

Thousands of people took shelter at the World Food Programme's compound, also close to the fighting, said Challis McDonough, the organization's senior regional spokeswoman.

#Women at antenatal clinic in #Juba Teaching Hospital taking cover from heavy shelling in town. #SouthSudan pic.twitter.com/4uQLEC7FQS — UNFPA South Sudan (@UNFPASouthSudan) July 11, 2016

The compound "is designed for about 100 people and it's got something like 3,000 in it right now," McDonough said.

She said the fighting might hinder distribution of food with the country's main center of coordination in lockdown.

"The humanitarian needs are acute in some parts of the country, and we are trying to make sure we can continue that support, but we need to make sure it is in a way that is as safe as possible for our staff and partners," she said.

A Kenyan nongovernmental organization worker trapped in a hotel talked to CNN online. CNN is not able to provide his name because he is not authorized to speak to media.

"We are stranded in hotels," he said. "All we can hear is the fighting outside. There is a perennial lack of supplies, food, water, fuel for the generators, and all we know is that the airport is closed and we cannot access it to fly out. The roads are being manned by soldiers who are robbing and harassing anyone who dares approach them.

"We keep thinking the fighting has stopped," he continued, "but even after two hours of calm, it is followed by hours of gunfire and shelling.

"Kenyans who are stuck here are ready to pay for their bus and air tickets out of Juba but the border is closed and airlines have canceled flights. The only way out is if fighting reduces."

'Sudden and serious'

The U.S. State Department said the security situation in Juba on Sunday had seen a "sudden and serious deterioration," with clashes between government and opposition forces breaking out into "general fighting."

Indian External Affairs Minister Sushma Swaraj said in a tweet that Indian nationals were being evacuated and warned others not to travel to the country. There are many Indian nationals working at the U.N mission there.

South Sudan - I am aware of the developments in South Sudan. We are planning evacuation of Indian nationals. /1 — Sushma Swaraj (@SushmaSwaraj) July 11, 2016

Koro Bessho, Japan's ambassador to the United Nations, confirmed the death of a Chinese soldier, while Chinese state broadcaster CCTV reported the death of a second Chinese peacekeeper.

Other Chinese and Rwandan peacekeepers also sustained injuries.

How many have been killed in fighting between the factions is unclear. One estimate puts the death toll close to 150, while other reports indicate more than 270 have been killed. CNN is working to confirm an exact death toll.

Gunfire from "heavy weaponry" was exchanged for much of Sunday outside a U.N. building on the outskirts of Juba, the U.N. mission to the country said.

The mission sent out a series of tweets at about 8:25 a.m. (1:25 a.m. ET) describing "gunshots" and a "heavily armed exchange" outside a U.N. compound.

The U.S. Embassy issued an alert saying that fighting between government and opposition forces was ongoing at the U.N. mission's headquarters, the Jebel area of the city and near the airport.

Sustained clash, ongoing since approx. 08:25 heavy weapons, UN House area @unmissmedia — UNMISS (@unmissmedia) July 10, 2016

The U.N. mission said that 1,000 internally displaced people had fled its protection as violence hit areas near their camps.

"Both UNMISS compounds in Juba have sustained impacts from small arms and heavy weapons fire.

"The United Nations urges all parties to respect the sanctity of the United Nations and condemns any deliberate targeting of United Nations premises and its personnel," it said in a statement.

Flights canceled

Kenya Airways, which operates two flights a day to Juba, said it was suspending all flights to the city because of an "uncertain security situation," while Britain's Foreign Office advised against all travel to South Sudan, saying "the security situation in Juba has deteriorated" since Friday.

Rwanda Air and Fly 540 also said they were suspending flights into Juba.

The country is nearly out of money because its funds come almost exclusively from oil revenue -- the value of which has plummeted. People have become desperate. In lieu of payment, government soldiers have reportedly been allowed to rape women, a U.N. report said

South Sudan gained independence in 2011 after 98% of the population voted to break away from Sudan. The East African nation, the youngest country in the world, quickly fell into civil war that took on ethnic undertones.

In December 2013, soldiers from Kiir's Dinka ethnic group tried to disarm Nuer soldiers perceived to be loyal to Machar. Soldiers targeted Nuer civilians in the ensuing fighting, Human Rights Watch said.

At least 50,000 were killed and more than 2 million displaced in the civil war, and nearly 5 million people faced severe food shortages.

Under a peace deal signed in August, Kiir became the president of the country, and Machar the first vice president.