* First such accusation since September deal
* Border bombing likely to further delay oil exports
* Rebels shell Sudanese border city in oil region
JUBA/KHARTOUM, Nov 21 Sudan and South Sudan accused one another of incursions in disputed border zones on Wednesday, jeopardising plans to secure the boundary and resume cross-border oil flows vital to their economies.
The two former civil war foes agreed in September to end hostilities and restart oil exports from the south through Sudan after coming close to war again in April, the worst violence since the south became independent in July 2011.
But relations have soured again in the past few days after both sides failed to agree on how to move their forces back from the unmarked joint border, something both had said must happen before oil exports resume.
South Sudan's army said Sudanese warplanes had bombed its side of the border, killing a civilian and wounding four others, - the first such accusation since the September deal.
"Sudan bombed our people in Northern Bahr al Ghazal. The bombing happened yesterday at 2 p.m.," army spokesman Philip Aguer said. "We were caught by surprise."
Sudan denied the accusation, saying its forces had attacked a camp some 40 km (25 miles) inside Sudanese territory used by rebels who are fighting the Khartoum government.
"The armed forces conducted an internal battle deep in its territory," Sudanese military spokesman Al-Sawarmi Khalid said in a statement on Wednesday.
"The presence of South Sudan's army in the area ... represents aggression and blatant military intervention on our land, and we have the full right to respond by dealing with them as enemy forces," he said.
Khartoum has long accused South Sudan of backing rebels of the Sudan People's Liberation Movement-North (SPLM-North) fighting the Sudanese government, something analysts find credible despite denials from Juba.
MUTUAL STAKES IN OIL
Sudan benefits from South Sudan's oil exports because Juba has to pay a fee for using northern pipelines and a Red Sea port. Juba halted production in January because of disputes over pipeline fees with Sudan.
South Sudanese President Salva Kiir said on Tuesday the resumption of oil production had been delayed after Sudan had made further demands.
Concerns about delays to reopening the oil pipeline pushed the Sudanese pound to a historic low at the start of the week, highlighting the need to restart oil flows, the main source of state revenues and dollars for both countries.
Fighting between Sudan and rebels in its southern border states of South Kordofan and Blue Nile has hampered the plan to set up a secure border zone.
On Monday, Sudanese state news agency SUNA said rebels from the Darfur region had set up camp in a contested border region.
Khalid said then that the rebels had been positioned just 10 km from the border in a disputed strip of land called Samaha, or Miles-14, that was a major obstacle during the September talks.
On another front, the SPLM-North, which seeks to topple Sudanese President Omar Hassan al-Bashir, shelled Kadugli, capital of South Kordofan, which has most of Sudan's oil reserves.
In a statement, the SPLM-North said it had attacked army positions inside Kadugli in self-defence after Sudan used Antonov planes to bomb rebel positions and villages.
"Five rockets landed in the city in the evening," one Kadugli resident told Reuters, asking not to be named. "People are frightened."
In another setback to the peace process, the United Nations said in a report that both countries had not yet opened the land border for trade, despite an agreement in September.
Analysts say while both Juba and Khartoum need the lucrative returns from oil, they also want to be seen as tough on the other side to shore up domestic support.