All six crew members aboard a Sudanese military helicopter have been killed after it crash-landed and burned in North Kordofan state, according to the army.

Fire broke out because of a "technical problem" aboard the Russian-made aircraft, three minutes after take-off from a base at El Obeid, the state capital, Sawarmi Khaled Saad, an army spokesman, told the AFP news agency on Friday.

The pilot crash-landed in a yard "but the fire destroyed the plane completely and all six crew were killed", he said.

Saad said the helicopter was carrying "military equipment" on a logistical mission.

The official SUNA news agency reported that the aircraft was a helicopter gunship.

El Obeid is a base for the Sudanese military but there is no known anti-government activity in that immediate area.

Sudan's armed forces since June have been battling fighters further south in adjacent South Kordofan state, as well as in Blue Nile state.

South Sudan's military spokesman said on Thursday that air raids killed 17 people in the border state of Western Bahr al-Ghazal, the second day of stepped-up bombing along the northern frontier.

The spokesman also said bombing had resumed over the previous two days around Jau, a disputed area along the South Kordofan-Unity state border.

Khartoum dismissed the allegations of bombing in Western Bahr al-Ghazal as "incorrect", and accused South Sudan of building up its troops in the Jau area to attack inside Sudan.

South Sudan separated from Sudan in July after an overwhelming vote for independence that followed more than two decades of civil war.

Each side has accused the other of supporting rebels inside its borders.