Rebels in Sudan's war-torn South Kordofan state claimed on Thursday to have shot down an air force Antonov bomber, which they said crashed in flames along the undemarcated border near South Sudan.

Neither Sudan's military spokesman nor independent witnesses in the remote area, where access for journalists is restricted, could be reached for comment.

Arnu Ngutulu Lodi, spokesman for the insurgent Sudan People's Liberation Movement-North (SPLM-N), said the plane, flying at a relatively low altitude, was hit by fire from a heavy machinegun late on Wednesday afternoon.

"They shot at it until they saw the wing burning," Lodi told AFP. "Some people in the mountain saw the plane burning."

The bomber finally crashed near Jau, more than 50 kilometres (30 miles) further south, on the disputed border with South Sudan but in South Kordofan territory, Lodi said.

"It was burning all night," residents there reported after hearing an explosion, he added.

Rebels were headed to the crash site to take photographs, he said, adding that this was at least the second Antonov downed by them since fighting began in June 2011.

The insurgents have reported an upsurge in government bombing and fighting since Sudan and South Sudan signed in September a deal for a demilitarised border buffer zone designed to cut support for the insurgency.

The ethnic and religious-minority SPLM-N belongs to an alliance of Sudanese rebels seeking to overthrow the Islamist Khartoum regime.

Lodi said the insurgents intercepted communications from the Antonov pilot who indicated his plane had been hit.

Rebels have previously reported the capture from government forces of .50 calibre "Dushka" and 12.7 mm heavy machineguns.

Before it was hit, the Antonov bombed rebel-held Kauda town, as well as the Heiban and Habilla areas, Lodi added.

Nuba Reports, an activist website of "citizen reporters" in South Kordofan's Nuba Mountains battleground, said a "low-flying" Antonov dropped bombs near a weekly market in Kauda on Wednesday afternoon.

A bomb which fell on Heiban killed a rebel soldier, Mohammed Sied, while wounding a second rebel and one civilian, Nuba Reports wrote.

Lodi said a civilian, not a rebel, was killed.

In a separate incident early on Wednesday, rebels said they ambushed the army and killed 10 soldiers on the main road north from Kadugli, the South Kordofan capital.

Analysts say casualty figures from either side in the war should be treated with caution.

SPLM-N were allies of southern insurgents during Sudan's 22-year civil war, which ended with a 2005 peace deal that led to South Sudan's independence in July last year.

Sudan has accused South Sudan of supporting the SPLM-N, a charge analysts believe despite denials by the government in Juba. The rebels are also fighting in Sudan's Blue Nile state.

More than 900,000 people are estimated to be displaced or severely affected by the fighting in South Kordofan and Blue Nile, the UN says, noting reports of serious food shortages and lack of adequate health care in rebel-held areas.

Sudan has cited security worries in tightly restricting the operations of foreign aid agencies and says there is no humanitarian crisis in the area.