* North's warplanes target airfield near Kauda -UN
* Northern army denies military actions kill civilians
* Refugee agency says aid work "severely constrained"
(Adds rights group report, edits)
By Alexander Dziadosz
KHARTOUM, June 14 (Reuters) - An air bombing campaign in Sudan's Southern Kordofan border state is causing "huge suffering" to civilians and endangering aid work in the region, the United Nations said on Tuesday.
The northern military has been battling southern-aligned groups it describes as rebels in the mountainous region for more than a week, raising fears of a mounting death toll after clashes escalated to include artillery and warplanes.
The fighting in Southern Kordofan -- the north's main oil state which borders south Sudan -- has increased tensions at a sensitive moment for Sudan, with the south set to declare independence in less than a month.
The south's main party has said the northern army sparked the fighting by trying to disarm southern-aligned fighters in the region ahead of the split. The north has accused the fighters of starting the conflict.
Analysts say Southern Kordofan could be the scene of protracted violence because it is home to thousands of armed and seasoned fighters who fought alongside the south against Khartoum during decades of civil war that ended in 2005.
"Intensive bombing by SAF in the past week is continuing in the surroundings of Kadugli and Kauda, where two jet fighters dropped 11 bombs this morning around 10:30 (0730 GMT), apparently targeting an airfield," U.N. Mission in Sudan (UNMIS) spokesman Kouider Zerrouk said, referring to the north's Sudanese Armed Forces (SAF).
He said two bombs fell close to the perimeter of a UNMIS compound located about 150 metres from the airfield.
"This bombing campaign is causing huge suffering to civilian populations and endangering humanitarian assistance," he said.
A northern army spokesman, Al-Sawarmi Khaled, denied Khartoum's military actions were killing civilians, saying fighting was only between the army and rebels. "There are not any victims from the civilian people," he said.
UNMIS called on all armed groups to "immediately allow access to humanitarian agencies and stop indiscriminate military attacks against civilians and protect them in accordance with international law".
The World Food Programme said in a report that up to 60,000 people may have fled the fighting so far.
AID ACCESS
A number of human rights agencies and church groups have accused northern security forces of conducting "house-to-house" searches and arresting or killing suspected political opponents in Southern Kordofan, charges Khartoum denies.
The Sudan Democracy First Group, a rights group, said on Monday it had documented dozens of cases of arbitrary arrests, torture and "extra-judicial killings," many targeting the state's ethnic Nuba population.
Rabie Abdelati, an information ministry official and senior member of the north's ruling National Congress Party, dismissed the charges as "political rumours," saying arrests were not made without justification and would go through the country's courts.
"I think this is something political and it has no weight," he said.
Also on Tuesday, the U.N. refugee agency urged Sudanese authorities to allow road and air access for aid workers trying to help thousands of people fleeing the fighting. [ID:nLDE75D1A9]
Humanitarian flights have been denied permission to land in the state capital Kadugli for nearly a week and roadblocks manned by armed militiamen have hampered land access, the United Nations High Commissioner for Refugees (UNHCR) said.
"Insecurity means our operations are severely constrained and UNHCR is currently unable to reach a warehouse just 5 km (2 miles) from the U.N. peacekeeping mission's base in the city," spokeswoman Melissa Fleming told a news briefing. Further underlining the deteriorating situation, the World Food Programme and the World Health Organisation said premises belonging to the two U.N. agencies in the area had been looted.
Southerners voted to secede in a January referendum which was promised in a 2005 peace deal that ended decades of north-south civil war. That conflict killed 2 million people.
The split, scheduled for July 9, has been complicated by a raft of unresolved issues, including where to draw the common border and how to divide oil revenues. (Additional reporting by Fredrik Dahl; Editing by Andrew Heavens)
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