* South official says Khartoum not ready to talk
* S. Sudan seceded in July with disputes left unresolved
* Fears border clashes could push states back to war
JUBA, May 21 (Reuters) - A date to re-start talks between feuding neighbours Sudan and South Sudan could be agreed this week, an African Union mediator said on Monday, but a senior South Sudanese official said he did not believe Khartoum was ready to negotiate.
Former South African President Thabo Mbeki is trying to bring the two Sudans back to the negotiating table at the AU's headquarters in Addis Ababa to try to resolve oil, security and border disputes that triggered an outbreak of border fighting between the two countries' armies last month.
South Sudan became independent in July 2011 under a 2005 peace agreement with Khartoum that ended decades of civil war. But the neighbours failed to mark their shared border and agree how much the landlocked South should pay to export its oil through Sudan.
The April clashes over an border oil zone raised fears the two states might plunge back into all-out war and prompted the United Nations Security Council to warn Juba and Khartoum earlier this month to negotiate peacefully or face sanctions.
Mbeki, who met Sudanese President Omar Hassan al-Bashir in Khartoum at the weekend, held talks on Monday in the South Sudanese capital Juba with South Sudan's President Salva Kiir and with chief South Sudanese negotiator Pagan Amum.
Mbeki said he was confident the two sides would heed U.N. and AU demands that they restart talks soon. "I am quite sure we can agree a date this week," he told reporters.
Sudan's Bashir said at the weekend he wanted a lasting peace with his southern neighbour but insisted security issues should be resolved first, specifically that South Sudan should end support for rebels in Sudan's borderlands.
El-Obeid Morawah, spokesman of the Sudanese foreign ministry, said Sudan wanted to wait for Mbeki to return to Khartoum on Tuesday for new talks with Bashir before deciding when negotiations could resume.
"We told Mbeki when he met President Bashir that we are ready to sit down for talks but security is a priority. This is also a priority for the African Union," he said. "If our neighbours in South Sudan agree on this then talks will resume as soon as possible."
South Sudan denies Khartoum's allegations that it supports rebels fighting the Sudanese army in the states of South Kordofan and Blue Nile, which both border South Sudan.
In comments in Juba which seemed to cast some doubt on Mbeki's apparent optimism, South Sudanese negotiator Amum accused Bashir of trying "impose preconditions" on the talks, a position which he said was a violation of a May 2 U.N. Security Council resolution calling on both sides to resume negotiations.
"It seems Khartoum is not ready to come (to talks) ... There is an implied refusal by Khartoum to commit to the negotiations without pre-conditions," Amum told reporters after Mbeki spoke.
Amum, who is secretary-general of the ruling Sudan People's Liberation Movement (SPLM) in South Sudan, has already urged the U.N. Security Council to impose sanctions on Sudan for what he says is Khartoum's violations of the May 2 resolution seeking an end to hostilities by both sides.
He repeated South Sudan's readiness to restart negotiations immediately. (Additional reporting by Ulf Laessing in Khartoum; Reporting By Pascal Fletcher; Editing by Andrew Heavens)
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