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AFP
Khartoum threatened retaliation on Sunday after accusing breakaway South Sudan of backing a rebel attack inside its territory, adding to tensions which have sparked international concern.

Rebels in a "revolutionary front" aimed at toppling the Khartoum regime claimed the attack in the disputed Jau area -- part of an oil-rich region on the poorly defined border -- as their first joint operation against government forces.

"It is a very big victory," said Arnu Ngutulu Lodi, of the Sudan People's Liberation Movement-North (SPLM-N).

The rebels claimed to have seized three tanks along with hundreds of weapons and vehicles in the joint operation with fighters from Darfur's Justice and Equality Movement (JEM).

"Joint forces managed to surprise them," he said.

Sudan's foreign ministry said rebels accompanied by officers from South Sudan's army launched a "direct attack" on the area six kilometres (four miles) inside the border, in violation of a memorandum on non-aggression and cooperation signed this month.

"Sudan reserves the right according to international law to react to this attack," the ministry said in a statement, adding that it will complain to the UN Security Council and African Union.

It is the latest flare-up over the Jau area and comes after UN chief Ban Ki-moon warned that the crisis between Sudan and South Sudan has become a major threat to regional peace and security.

Britain this month expressed "grave concern at the recent build-up of forces and escalation of tensions in conflict-affected border areas."

The SPLM-N has for several months been fighting in South Kordofan and Blue Nile states along the border with South Sudan.

One analyst has dismissed the Revolutionary Front as "just a name," but Lodi said the joint attack -- albeit with a small JEM component -- showed their commitment to work together.

"We are not just talking. We are doing it," he told AFP.

Last November, both rebel groups joined with factions of Darfur's Sudan Liberation Army to form the front dedicated to "popular uprising and armed rebellion" against the government in Khartoum.

"This attack is under the umbrella of the Sudanese Revolutionary Front," JEM spokesman Gibril Adam Bilal told AFP.

Sudan Armed Forces spokesman Sawarmi Khaled Saad confirmed that the area had come under attack, but he blamed the forces of South Sudan.

"This attack was completely planned and sponsored by the government of South Sudan," he said. "The fighting is going on now."

Neither side could immediately give casualty figures.

Access to the state is restricted, making independent verification difficult.

In December, Sudan accused the South Sudan army of attacking the Jau region, but Juba's military insisted its troops were defending an area on their side of the frontier.

Two weeks ago, Juba said Khartoum had bombed the same area from the air, violating the memorandum on non-aggression and cooperation.

Washington has condemned the "unjustified and unacceptable" bombing of civilians by the Sudanese military in South Kordofan and Blue Nile.

South Sudan broke away in July last year after an overwhelming vote for independence following more than two decades of war that killed two million people.

Border tensions have since flared, with each side accusing the other of supporting rebels within its territory, while a major dispute over oil transit fees remains unresolved.

Asked about Khartoum's latest accusations, Juba's information minister, Barnaba Marial Benjamin, said the rebels were Sudan's problem.

"We don't give any support to opposition groups in Sudan," he said.

The ethnic minority insurgents from SPLM-N had previously fought alongside the former rebels now ruling in Juba.

Lodi, the SPLM-N spokesman, said Sunday's joint attack followed formalisation last week of the Revolutionary Front's structure, and vowed that more combined operations will occur "when appropriate".

JEM and SPLM-N had fought together once before, in August.