Sudanese insurgents said they brought down a military helicopter and attacked a government convoy when the army's chief of staff visited a flashpoint in Sudan's main oil region on Friday, rebels said.
A witness also reported seeing clashes, but the government denied a helicopter had been brought down, or that there had been any fighting.
Gibril Adam, spokesman for the Justice and Equality Movement (JEM), one of the biggest rebel groups, said its fighters had attacked a government convoy carrying Sudan's army chief of staff, Ismat Abdelrahman, just outside Abu Kershola.
"We managed to shoot down a helicopter and hit another one," Adam said, adding that nine people had been killed in fighting.
Sudan's army spokesman, al-Sawarmi Khalid, said a military plane had crashed near Abu Kershola, a town in South Kordofan state which the army seized this week after rebels said they had withdrawn.
But he denied clashes took place and said the plane had crashed due to a technical fault. "It was on a routine mission in the Abu Kershola area," he said.
A witness told Reuters heavy shooting broke out after rebels opened fire on the convoy and the army had used tanks and a helicopter, which apparently was hit.
"I saw a helicopter in the air clouded in smoke," the witness said.
The Sudanese Revolutionary Front (SRF), an umbrella of rebel groups to which JEM belongs, had launched an assault on several towns late in April, occupying Abu Kershola until it said it had pulled out.
Abu Kershola lies at the northeastern tip of South Kordofan, close to White Nile state, home to the country's main Nile river port and sugar industry.
The violence has strained relations with South Sudan, which Sudan accuses of backing rebels who complain of marginalisation by the Khartoum elite. Juba denies the claims.