Ethiopian Prime Minister Hailemariam Desalegn (right) looks on as South Sudan's rebel leader Riek Machar (centre) and South Sudan's President Salva Kiir (left) exchange signed peace agreement documents in Addis Ababa. File picture: Goran Tomasevic

Khartoum -

The crisis in South Sudan is causing regional instability, neighbouring Sudan warned on Thursday, expressing “great concern” as fierce battles raged south of the border.

Rebel and government forces in the South traded blame for breaking a ceasefire deal signed last week, the second time since the civil war began in mid-December that a truce has failed to stick.

“Sudan is following with great concern reports that both sides of the conflict violated the agreement signed in Addis Ababa on May 9,” foreign ministry spokesman Abubakr al-Siddiq said in a statement.

The war began when South Sudan's President Salva Kiir accused his former vice-president, Riek Machar, of attempting a coup.

Since then, thousands and possibly tens of thousands of people have died, while more than 1.3 million people have been forced to flee their homes.

Almost 80 000 have headed north into Sudan, where the aid system was already struggling to cope with more than six million other people needing assistance, according to the United Nations.

“The crisis in South Sudan has a great impact on all the regional countries, especially Sudan,” Siddiq said.

“No country can have stability while its neighbours suffer from fighting and insecurity.”

Both combatants on Thursday reported heavy fighting in the South's oil-producing state of Upper Nile, which now pumps almost all of South Sudan's crude after fighting shut most fields in Unity state.

Southern crude passes through Sudan for export, and both impoverished nations have lost vital revenue as a result of the cut in production.

The South separated from Khartoum's jurisdiction three years ago under a 2005 peace deal that ended 23 years of civil war. - Sapa-AFP