Ethiopia on Monday called for the withdrawal of all foreign forces, notably Ugandans, from South Sudan, warning of a threat of regional conflict.

"Because of this intervention, the conflict might end up as a regional conflict because there are other interests also from other sides," Prime Minister Hailemariam Desalegn told reporters.

South Sudan has been embroiled in deadly fighting since mid-December when clashes between forces loyal to President Salva Kiir and former vice president Riek Machar erupted in the young nation.

Kiir asked Ugandan forces to protect key infrastructure in Juba, including the airport and the presidential palace, but the presence of Ugandan troops has been fiercely condemned by Machar and his allies.

The warring parties signed a shaky ceasefire on January 23 that included a call for the incremental exit of all foreign forces, a major sticking point in the peace talks that kicked off in the Ethiopian capital early last month.

Hailemariam called for the withdrawal of alien forces in order to end the conflict that has killed thousands and displaced 900,000 people.

"I hope for the cessation of hostilities..., Ugandan forces and all other external forces must withdraw from that area phase by phase," he said.

Despite the agreement signed last month, sporadic fighting has continued in South Sudan, where aid agencies have warned of a deepening humanitarian crisis.

A new round of peace talks was set to begin Monday between government and opposition parties but was delayed until Tuesday for logistical reasons, mediators said.

Rebels threatened to block the talks, demanding the withdrawal of Ugandan troops along with the release of four political detainees arrested after fighting erupted on December 15.

The latest round of talks, mediated by regional bloc IGAD, is aimed at addressing the roots of the crisis in order to forge long-term political solutions.

Oil-rich but deeply impoverished South Sudan gained independence from Sudan in July 2011 following decades of civil war.