Former South African President and current African Union mediator Thabo Mbeki met with South Sudan's President Salva Kiir on Thursday in Juba. "We have come to Juba essentially to see President Salva Kiir to seek his advice about the next steps that we should take with regards to the relations between South Sudan and Sudan," the former South African president said.

"We wanted to discuss that with President Kiir before we proceed to Khartoum as part of the preparations for that summit," Mbeki said, referring to a summit by Sudanese President Omar al-Bashir and Kiir initially set for 3 April but postponed after a recent border flare-up.

Mbeki, who was accompanied by former Burundian president Pierre Buyoya, also on the mediation team, said he would later travel to Khartoum for talks with Bashir.

Juba and Khartoum negotiators failed to sign an agreement on security after the latest round of AU-mediated talks wound up Wednesday in Addis Ababa — the first face-to-face meeting since border clashes flared late last month.

South Sudan's lead negotiator Pagan Amum accused the Khartoum delegation of walking out of the talks, saying "war mongering" prevented them from signing the agreement.

He also said South Sudan's army downed a Sudanese fighter jet over a border area on the southern territory, but Sudan rejected the claim as well as the accusation that it refused to sign a deal.

The Khartoum delegation said it had to return home for consultation before committing to the accord.

Talks are to resume in the Ethiopian capital next week.

The recent border clashes involving air strikes and ground battles were the worst since South Sudan gained independence from Sudan last July, sparking international fears of a full-blown war.

Juba and Khartoum have traded blame over who started the clashes in the oil-rich Heglig region close to their undemarcated border.

In February, the two sides signed a non-aggression pact but it has been repeatedly violated in recent weeks.

World leaders have called on the two nations to end the hostilities. The United States on Wednesday voiced concern over the recent violence.

"We're calling on restraint on the part of all sides. And we're very concerned about the ongoing hostilities on the border areas between Sudan and South Sudan," State Department spokesman Mark Toner said.