They said the suicide bomber struck when the two buses travelled on the road between the border town of Rafah and the coastal city of el-Arish. The explosion damaged both buses, the officials said. The 10 victims were the bus's driver, three members of a security detail and six of the off-duty soldiers, according to a statement by Col. Mohammed Ahmed Ali, a military spokesman.
"The precious blood of our sons strengthens our resolve to cleanse Egypt and shield its sons from violence and treacherous terrorism," Ali wrote on his Facebook page. The wounded were being treated in military hospitals, he said.
The soldiers belong to the 2nd Field Army, which is doing most of the fighting against Islamic militant waging an insurgency against security forces in Sinai. The buses were on their way to Cairo, the officials said.
The officials spoke on condition of anonymity as they weren't authorized to release the information.
The northern Sinai region, which borders Gaza and Israel, has been restless for years, but attacks have grown more frequent and deadlier since the July ouster of Islamist president Mohammed Morsi.
Latest in series of attacks
There was no immediate claim of responsibility for Wednesday's attack, but suicide car bombings are a signature method by militant groups linked to or inspired by al-Qaeda. It was the latest in a series of similar attacks targeting army and police facilities and checkpoints. In August, gunmen pulled 25 police conscripts off minibuses in the Sinai and shot them dead on the side of the main road linking Rafah to el-Arish.
Northern Sinai's violence occasionally has spilled over into cities in the southern part of the peninsula as well as mainland Egypt, targeting policemen, soldiers and politicians.
On Wednesday at dawn, assailants threw a grenade at a police checkpoint in a northern Cairo suburb, injuring four policemen, security officials said. The assailants fled the scene, the officials said.
In September, the Interior Minister, who is in charge of the police, survived an assassination attempt by a suicide car bomb. Earlier this week, a senior security officer who monitors Islamist groups, including Morsi's Muslim Brotherhood, was shot dead as he drove in Cairo's eastern Nasr City district.
Nasr City is a Brotherhood stronghold and home to several military barracks.