* Weeks of border clashes have raised fears of war
* Africn Union deadline for talks expires on Tuesday
* Sudan extends deadline for expulsion of S.Sudanese
By Yara Bayoumy
KHARTOUM, May 3 (Reuters) - Sudan and South Sudan's armies, who have been clashing along their disputed border for weeks, said on Thursday the frontier was calm after both accepted a U.N.-backed African Union plan to cease fire.
The crisis between the former civil war foes has raised fears of a return to all-out conflict and that prompted the AU to issue an ultimatum for both to stop fighting with the Security Council threatening sanctions if they did not comply.
"There are no military operations at the border. There is an atmosphere of guardedness and watchfulness," Sudanese army spokesman Al-Sawarmi Khalid said on Thursday. "The situation has been quiet for the last two days. The Sudanese army has not announced a formal ceasefire, but the fronts are quiet."
Philip Aguer, South Sudan's army (SPLA) spokesman, also said the frontier, along which clashes between rebels backed by Khartoum and by Juba have also broken out, was calm. "It has been quiet on the border for the last 48 hours," he said.
After several rounds of failed talks to resolve disputes over oil exports, border demarcation and citizenship, the tensions escalated during the past month into fighting along the ill-defined, 1,800 km (1,200 mile) frontier.
At one point South Sudan, which seceded and declared independence from Sudan last year, seized a disputed oilfield in Sudan, before withdrawing under international pressure.
Khartoum's warplanes have bombarded several areas in the oil regions of the South's Unity state.
Sudan has denied carrying out specific air strikes, but has reserved the right to use aerial attacks to defend territory.
The conflict has halted nearly all oil production in both countries, the lifeline of their respective economies.
MUTUAL PLEDGES TO SEEK PEACE
South Sudan said it had accepted the AU's seven-point roadmap that called for a cessation of hostilities, while Sudan said it had assented to the plan "in principle".
Sudan said it had confidence in the AU's mission and that it was committed to cooperating with it.
The foreign ministry also said Khartoum was committed to a long-standing peace with the South and hoped Juba would cooperate with the AU and Security Council decisions.
Both countries deny the other's allegations of supporting rebel forces.
The U.N. Security Council on Wednesday unanimously passed a resolution threatening Khartoum and Juba with sanctions if they failed to silence the guns and resume talks within two weeks, endorsing the AU's deadline of May 8.
The AU's plan also calls for Sudan and South Sudan to withdraw troops from contested areas and resume talks with the aim of resolving all outstanding disputes.
Neither can afford a protracted, full-blown conflict but distrust runs deep between them, especially after the South gained independence from its northern neighbour last July under a 2005 peace deal that ended two decades of civil war.
The breakaway left tens of thousands of South Sudanese stranded as foreigners in Sudan. Sudan's government initially gave them until April 8 to get the right papers or leave.
The Sudanese government issued a May 5 departure deadline for up to 12,000 South Sudanese stuck in the port of Kosti, hoping to leave by Nile barge. But Khartoum then halted river traffic because it said South Sudan was using it to transport weapons to rebels.
The May 5 deadline prompted an outcry from the United Nations and the International Organisation for Migration. The government said late on Wednesday the South Sudanese now had until May 20 to leave. (Additional reporting by El-Tayeb Siddig; Writing by Yara Bayoumy; Editing by David Clarke)