Khalil Abdallah, political leader of the Darfur rebel National Movement for Reform and Development (NMRD), said 17 soldiers were taken prisoner in the attack on the town of Arm Yakui, some 30 km (19 miles) northwest of West Darfur's main town el-Geneina.
A Sudanese army source confirmed there was an attack on one of their bases in the area but could not give casualty figures.
"We attacked the town of Arm Yakui today -- a Sudanese military base," Abdallah told Reuters.
"We killed 78 soldiers and took 17 prisoners," Abdallah said, adding his group lost two men with five injured.
Darfur rebels took up arms in early 2003 accusing Khartoum of neglect and of arming Arab militias to loot and burn non-Arab villages. Tensions escalated in the past few months as neighbour Chad accused Sudan of harbouring Chadian guerrillas trying to oust President Idriss Deby.
Two other Darfur rebel groups are in peace talks with Khartoum, but the NMRD are not and do not respect a ceasefire signed between those groups and the government in 2004.
Tens of thousands have been killed and more than 2 million forced from their homes during the Darfur violence which Washington calls genocide, a charge Khartoum rejects.
The Sudanese army source said the attack came from within Chadian territory.
"This attack came suddenly from inside Chadian territory, and we returned fire with the same force using artillery," he said. He said he had no further information from the remote region.
The NMRD operate along the Chad-Sudan border. The long border between Chad and Sudan is porous and many tribes span the frontier. Deby himself took power in 1990 in an uprising he launched from Darfur.
Abdallah said Chadian rebels, led by Mahamat Nour, had fought alongside the Sudanese armed forces in the attack.
"We don't understand why they are doing this. We have no problem with Mahamat Nour," he said.
Nour leads an alliance of Chadian insurgents called the United Front for Democratic Change, known as FUC. His group attacked the border town of Adre in December and are sworn to depose Deby.