The 'Troika' of the United States, Britain, and Norway have expressed their 'deep concern' that South Sudan's leaders failed to form a transitional government within 90 days of signing a peace agreement, as the deal demanded.

The group of donor countries who have long engaged with South Sudan urged the authorities on all sides to establish a transitional government now and recommit to the timeline of the agreement, saying the peace process could otherwise unravel.

"Each day, the fighting and abuses continue, and an already grave humanitarian situation grows worse," the group said in a joint statement.

The three nations called on members of the armed opposition and the former detainees to return to Juba to attend tomorrow's inaugural meeting of the Joint Monitoring and Evaluation Commission, an international group which includes members of the warring parties tasked with overseeing and implementing the peace agreement.

The Troika said that whoever does not participate in the meeting will be responsible for further delays of the implementation of the peace agreement, which would prolong the suffering of the South Sudanese people.

"It is vital that all parties participate fully [in the meeting]," the Troika said.