By Ulf Laessing

ADDIS ABABA, Sept 24 (Reuters) - Sudan and South Sudan leaders will try again on Tuesday to seal a border security deal after failing to achieve a breakthrough in the previous two days, officials said on Monday as both sides disagreed over whether progress had been made.

Sudanese President Omar Hassan al-Bashir and southern counterpart Salva Kiir have been meeting in Ethiopia since Sunday in hopes of wrapping up peace talks after coming close to all-out war in April.

The African Union has been striving to broker a demilitarised buffer zone at the unmarked and disputed border to allow South Sudan to restart oil exports through the north, which would give a big lift to both battered economies.

Both nations are under pressure after a U.N. Security Council deadline expired on Saturday, although it was extended unofficially until Thursday when AU mediator Thabo Mbeki will report on whether a deal has been done, diplomats said.

Bashir and Kiir originally planned a one-day summit but have so far failed to agree during three long meetings on the details of a buffer zone map proposed by the AU.

Sudan has reservations about a 14-mile-(23-km)-long strip of land mapped out, while Juba has accepted the formula.

Sudan said it was hopeful that Bashir and Kiir would successfully conclude the talks on Tuesday.

"Both leaders will discuss the issues tomorrow at 10 o'clock ... We expect all issues to be solved positively," El-Obeid Morawah, spokesman for Khartoum's foreign ministry, told reporters after Bashir and Kiir met for two hours.

"There (was) progress today regarding the issue of 14 miles and the controversial issues were minor today compared to other days," he said.

SOUTH SEES OBSTACLES

But South Sudan's delegation appeared more doubtful.

"We have been facing some obstacles during the presidents' discussions. We haven't been able to solve this yet but tomorrow a final session will be held," Atif Keir, spokesman for Juba's delegation, told reporters.

He said the talks would only succeed if Sudan complied with a road map for peace proposed by the African Union, which has been trying to broker a comprehensive peace deal.

The armies of both nations fought for weeks in April along the unmarked and disputed border after a dispute escalated over how much South Sudan should pay to use oil pipelines that traverse the north to Red Sea ports.

The two reached an interim deal in August to restart oil exports from landlocked South Sudan after Juba turned off its wells in a row over export fees.

But Sudan insists on first reaching a security accord.

South Sudan, where most people follow Christianity and animism, seceded from the mainly Muslim north in July 2011 under a 2005 peace agreement that ended decades of civil war.

Western and African officials had hoped for broad peace accord but several rounds of talks in Addis Ababa have brought no visible progress on settling the fate of five disputed border areas. This will probably be left to a future round of negotiations or possible lengthy arbitration. (Editing by Mark Heinrich)