Sudanese air force bombs have hit a Doctors Without Borders hospital in South Kordofan, the group said Tuesday, adding to Western concerns about attacks against civilians in a largely-hidden war.

Paris-based Medecins Sans Frontieres said its facility in Farandalla, South Kordofan, was "attacked".

Sudan's military denied the incident, the second case of its kind involving a hospital in the past few weeks.

"As bombs struck the village of Farandalla on June 16, two hit the MSF hospital there. Five people were wounded in the village and one MSF staff member was injured at the hospital," the international aid group said in a statement.

"We are shocked that a medical facility can be bombed, especially since it was clearly identified with a flag and a cross on the roof," said MSF head of mission Brian Moller. "We also had previously communicated the hospital's position to the authorities in Khartoum."

Sudan's military spokesman Sawarmi Khaled Saad told AFP no air force planes had been used in the area.

"We did not attack any hospital there because our target is not civilians," he said.

Bombs destroyed the emergency room, a dressing room, the pharmacy, and the hospital kitchen, MSF said, describing damage as "significant".

"But MSF will continue to work there," Moller said.

The US ambassador to the United Nations last week accused Khartoum of intensifying attacks on civilians in South Kordofan and Blue Nile states.

Ground and air strikes have increased since April, with hundreds of barrel bombs and other ordnance dropped on towns and villages, deliberately targeting hospitals and schools, Ambassador Samantha Power said.

The United States was also disturbed by reports of air strikes targeting civilian aid workers, which if accurate would seriously violate international law, she said.

The United Nations Office for the Coordination of Humanitarian Affairs reported in its May 26-June 1 weekly bulletin that 59 bombs had been dropped around the rebel stronghold of Kauda.

Citing reports from international and local aid groups, it said "the aerial bombings hit a number of civilian structures, including an office of a local aid organisation, a market, an orphans' school, and a former hospital."

In a joint statement last week, Washington, Britain and Norway said Khartoum "continues to wage war and target civilians in Southern Kordofan and Blue Nile and bears primary responsibility for intensifying the conflict in Darfur".

In early May the US and Canada said they were "appalled" by an air raid which Sudanese forces reportedly carried out near a church-run hospital in Kordofan.

They said a number of bombs were said to have been dropped by Sudan's air force near the Mother of Mercy Catholic Hospital in South Kordofan's Nuba Mountains, which have born the brunt of the three-year-old war.

Some people were injured.

Sudan's military said that incident also did not occur.

MSF is one of the few healthcare providers in South Kordofan, where the movement of aid workers and others has been restricted since government forces and the rebel Sudan People's Liberation Movement-North (SPLM-N) began fighting.

Like the 11-year-old war in Sudan's western Darfur region, the Kordofan conflict has been fuelled by complaints among non-Arab groups of neglect and discrimination by the Arab-dominated regime.

More than one million people in government-held areas of South Kordofan and Blue Nile have been affected by the war, says the UN, which has no access to rebel zones and cannot verify the humanitarian impact there.

President Omar al-Bashir and Defence Minister Abdelrahim Mohammed Hussein are both wanted by the Hague-based International Criminal Court for alleged war crimes in Darfur.