At least 25 Egyptian policemen were killed when suspected militants ambushed a personnel carrier near Rafah - the crossing on the Israeli-Egypt border.
Egypt's interior ministry and state television reports, also, that two policemen were wounded in the attack. Another conscript died from wounds hours after the assault.
The security personnel were attacked while on their way to their barracks in Rafah.
Stories conflict on the deaths. The interior ministry said armed assailants had opened fire on the bus and state television reported rocket-propelled grenades were fired.
Security officials, speaking on condition of anonymity, told Ahram Online it was an execution-style murder.
Sources told Ahram Online that the army's Chief of Staff Lieutenant General Sedki Sobhi and Interior Minister Mohamed Ibrahim have left to Rafah following the attack.
Sinai has been witnessing almost daily attacks targeting security forces by suspected militants since the 3 July ouster of Islamist president Mohamed Morsi.
Egypt's military and security forces have been engaged in a long battle against militants in the northern half of the peninsula. So far, tens of police and army conscripts and officers have been killed.
In 2012, an August attack by unknown assailants killed 16 army conscripts in Rafah. Then-president Mohamed Morsi made a security reshuffle that saw army intelligence chief Abdel-Fattah El-Sisi become defence minister.
El-Sisi removed Morsi from power on 3 July after mass protests calling for early presidential elections.
Militants and tribesmen have been known to use Sinai for arms smuggling and other criminal activity. Militants have fired rockets into Israel and staged other cross-border attacks there on several occasions.