Gunmen who launched a deadly assault on an Egyptian border post near Israel on Sunday were backed by mortar fire from the Gaza Strip, the Egyptian army said.

"At the time of the attack, elements in the Gaza Strip helped by firing mortars at the area of the Karm Abu Salem" border crossing, the army said in a statement.

The army said 35 gunmen were involved in the "terrorist act" which it added was launched during "the iftar and the call to prayer" signalling the end of the daytime Muslim fast during Ramadan.

The gunmen, who are suspected of being jihadists, attacked a border crossing between Egypt and Israel on Sunday, killing 16 guards, before entering the Jewish state where Israel says its forces killed five of them.