Sudan's president has turned down an invitation from the African Union to meet South Sudan's leader on Wednesday to move forward stalled talks to end hostilities, state news agency SUNA said on Tuesday.

The neighbours came close to war when border fighting escalated in April, the worst violence since South Sudan declared its independence a year ago under a 2005 agreement that ended decades of civil war.

African Union-sponsored talks between negotiators from both sides have ground to a halt over disputed issues, including where to mark the border and how much landlocked South Sudan should pay to export oil through northern pipelines.

The two countries face a possible threat of sanctions from the U.N. Security Council if they do not resolve their disputes by Thursday.

Former South African President Thabo Mbeki, the AU's mediator, invited Sudanese leader Omar Hassan al-Bashir to meet South Sudan President Salva Kiir in Ethiopian capital Addis Ababa on Wednesday, Sudan's foreign ministry told SUNA.

But Bashir turned down the meeting because of a previously scheduled visit to Qatar, the ministry said.

"The government prefers that such a summit should be held after good preparation and planning," ministry spokesman El-Obeid Morawah told the state news agency.

He said a presidents' summit "should not discuss details of negotiations, but finalise certain issues to get positive results for the situation of both countries."

Speaking in New York after the U.N. Security Council renewed the mandate for the U.N.-African Union peacekeeping force in Darfur, Britain's U. N. ambassador, M ark Lyall Grant, t old reporters it was crucial for Khartoum and Juba to meet the Thursday deadline for results of their dialogue.

"We had hoped that the two leaders would be getting together early this week in order to try and resolve those outstanding differences," Lyall Grant said.

"We urge both leaders to show the necessary statesmanship to make the necessary compromises so that agreement can be reached on these outstanding issues," he said. "The Security Council has set a deadline and expects results by that deadline."

Given Russia's and China's opposition to sanctions, however, council diplomats said the council was unlikely to do anything more than issue a statement rebuking Sudan and South Sudan if they fail to meet the Thursday deadline as expected.

The two sides have made progress on oil talks, but have still not reached an agreement, Morawah said. Talks over the disputed border will resume on Wednesday, he said.

There was no immediate comment from South Sudan or the African Union.

South Sudan shut down its oil production in January after both sides failed to agree on an export fee and Khartoum started taking oil as what it called compensation for unpaid fees. Oil is the lifeline of both economies.

Sudan accuses South Sudan of supporting rebels in two southern border states and the Western Darfur region, allegations that some diplomats find credible despite denials from Juba.

South Sudan itself accuses its neighbour of bombing its territory. Sudan routinely denies these allegations but Reuters reporters have witnessed several air strikes.