The security situation in the arid peninsula has become increasingly precarious since the 2011 uprising.
At least 10 Egyptian soldiers were killled in a car-bomb attack in the volatile border region of northern Sinai, security officials say.
They said the car bomb on Wednesday struck a bus carrying the off-duty soldiers as it travelled on the road between the border town of Rafah and the coastal city of el-Arish. Another 35 soldiers were wounded in the attack.
In a statement on Egyptian television, the head of the army, General Abdel Fattah al-Sisi, said that the bombing would “strengthen our determination” and that anyone who raised arms against the army and the police was a “terrorist”.
State TV said three days of mourning had been declared.
The soldiers belong to the 2nd Field Army, which is doing most of the fighting against armed groups waging an insurgency against security forces in Sinai.
The bus was on its way to Cairo, the officials said.
There was no immediate claim of responsibility for Wednesday’s attack, but suicide car bombings are a signature method by armed groups linked to or inspired by al-Qaeda.
The northern Sinai region, which borders Gaza and Israel, has been restless for years, but attacks have grown more frequent and deadlier since the overthrow in July of the Islamist president, Mohamed Morsi.
Wednesday’s car bombing was the latest in a series of similar attacks targeting army and police facilities and checkpoints.
In August, armed men pulled 25 police conscripts off mini-buses 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.