* Both sides at odds over border, oil
* Clashes in April threatened all-out war
* Defence ministers attend talks (Adds fresh accusations from South Sudan)
By Aaron Maasho
ADDIS ABABA/KHARTOUM, June 4 (Reuters) - Sudan and South Sudan on Monday began their first direct high-level talks on border security since a series of frontier clashes threatened to drag the former civil war foes back into a full-scale conflict.
Perched atop some of Africa's most significant crude reserves, the two countries have been mired in disputes over oil revenues and demarcation of their border since South Sudan gained independence in July.
The two countries returned to African Union-mediated talks last week, after the United Nations threatened to impose sanctions if they failed to stop fighting along the border and hammer out a deal.
"We are here for the joint political and security mechanism meetings - the body ... that is primarily drawing up the safe and demilitarised border zone," South Sudan's Foreign Minister Nhial Deng told Reuters.
"We are always optimistic, you have to because it is optimism that fuels hope and hope helps you achieve success."
The defence and interior affairs ministers of both countries were also attending the negotiations, as well as military figures, officials said.
Tensions are still running high. South Sudan's chief negotiator Pagan Amum accused Sudan of keeping some security forces in a disputed border region despite pulling out most of its forces to pave the way for talks.
South Sudan has also accused Khartoum of launching repeated air strikes on its territory. Juba announced on Friday it had filed a complaint at the U.N. Security Council.
Khartoum regularly denies accusations it is bombing South Sudan's border states, some of which are oil-producing. Such accusations are hard to verify as the remote area is difficult to access.
The talks had been cut short after South Sudan seized the Heglig oil field in a disputed border region in April, only to withdraw later under heavy international pressure.
SECURITY FIRST
South Sudan has criticised Sudan for insisting on discussions on security ahead of other issues, in defiance of the U.N. peace plan.
Sudanese officials denied making preconditions.
"The meetings will kick-start this afternoon and we are hopeful these issues will be addressed in a very genuine and action oriented way," said Omer Dahab, spokesman of Khartoum's delegation.
Sudan paved the way for the resumption of talks on Friday after it announced the withdrawal of its security forces from the disputed Abyei region, as demanded by the United Nations.
But South Sudan negotiator Amum said Khartoum was still keeping around two battalions of national intelligence and security forces in Abyei's oil fields.
"They (Security Council) confirmed the withdrawal of armed forces from Abyei town and ... the police forces have also been withdrawn. Now we are having what they call security forces which are part of the armed forces of Sudan. They must withdraw as per the resolution," he said.
There was no immediate response from Khartoum.
Abyei, seized by northern troops last year, is a major bone of contention between the two countries. It has fertile grazing land and some oil reserves.
In Khartoum, the head of the Sudanese side of the joint Abyei administration accused members of the southern Dinka tribe, which is allied to Juba, of trying to cause chaos in the region after the army's withdrawal.
"There were some provocations and property of citizens was looted," Khair al-Fahim told reporters in the Sudanese capital Khartoum.
"We call on the U.N. peacekeepers to stop these provocations," said Fahim who belongs to the Arab Misseriya tribe, which is allied to Khartoum.
South Sudanese overwhelmingly voted to secede from Sudan in a referendum last year, promised in a 2005 peace deal that ended decades of north-south civil war.
The new, landlocked South inherited most of the old united Sudan's known oil reserves. But it shut down production in January to stop Khartoum taking oil for what the latter called unpaid export fees. (Additional reporting by Ulf Laessing; Writing by Aaron Maasho and Alexander Dziadosz; Editing by Rosalind Russell)
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