The United Nations says that three-quarters of South Sudan’s population is in need of humanitarian aid following several days of internal conflict.

On Tuesday, the director of the UN World Food Program, Etharin Cousin said, "This latest conflict is going to push even more people into hunger and despair."

Fighting broke out in the capital Juba on Thursday, when troops loyal to South Sudan's President Salva Kiir clashed with Vice President Riek Machar’s forces.

On Monday, Kiir issued directives for an immediate cessation of hostilities, and Machar called on his loyalists to observe the ceasefire. The fragile ceasefire still appeared to be holding on Tuesday.

"Three quarters of the population of South Sudan is in need of humanitarian assistance," Cousin was quoted by AFP as saying.

She noted that due to the heavy fighting humanitarian staff had been forced to take shelter in underground bunkers "because the shelling was in Juba and so close to our places of operation.”

"We had over 2,000 people taking shelter inside our compound with our staff," she added.

Earlier, the UN announced that at least 36,000 people have been displaced since fresh heavy fighting erupted in the country’s capital.

A young boy orphaned by the recent fighting in South Sudan on July 12, 2016 in Juba (AFP)

Thousands of people have been killed and more than three million forced to flee their homes in the conflict that started in December 2013, when President Kiir sacked Machar, his former deputy, only two years after the country seceded from Sudan.

The two sides eventually signed an agreement in August last year to bring the conflict to an end. As part of the deal, Machar returned to Juba in April to take up the post of the first vice president in a national unity government.

Despite the August 2015 peace deal, battles persist across the country. There are numerous militia forces that do not abide by peace agreements and are driven by local agendas.