The leaders of Sudan and South Sudan must agree at their summit on Tuesday to block a one-sided referendum in the flashpoint Abyei region, a tribal chief said.

Mukhtar Babo Nimir leads the Arab Misseriya pastoralists who move seasonally through Abyei with their cattle.

He told AFP that his group also has the option of holding its own unilateral referendum if the rival Ngok Dinka go ahead with plans for a ballot.

"We are waiting for the results of President Bashir's visit to Juba today," Nimir said as Omar al-Bashir arrived in the South Sudanese capital for talks with his counterpart Salva Kiir.

"If he comes back with a deal for the Dinka to stop the referendum process, which they've started, that will be good," he said.

The Ngok Dinka, closely connected to South Sudan, are permanent residents of oil-producing Abyei.

At a 'People's General Conference' held last Friday, they declared their intention to hold a referendum to resolve the status of Abyei but set no date.

Abyei was meant to vote whether it lies in Sudan or South Sudan in January 2011 -- the same day as Juba voted overwhelmingly to split from the north -- as part of the 2005 peace deal which ended Sudan's two-decades long civil war.

But that vote never happened and Sudanese troops stormed the enclave and occupied it until May 2012. Since then, the impoverished area has been in political limbo.

The United Nations, which is the only authority in the area, has previously warned that any unilateral move would risk inflaming tensions in war-ravaged Abyei.

On Monday, the African Union reiterated that its own proposal for a referendum this month, to determine whether Abyei belongs with Sudan or South Sudan, is "a fair, equitable and workable solution."

That ballot has been stalled over disagreement about voter eligibility.

After a summit in September, diplomats said Kiir and Bashir seemed to have reached a deal to avoid an immediate official ballot on Abyei.