Gunmen killed 21 Egyptian soldiers at a security checkpoint on Saturday.
Officials said a weapons storage facility was blown up by a rocket-propelled grenade during the attack in Wadi al-Gadid governorate in western Egypt.
Security officials said the attackers were smugglers but an army spokesman said the assailants were "terrorists".
Two of the gunmen were killed in the clashes.
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The US president is set to call on leaders to rally together in the face of threats from North Korea and Iran at the UN General Assembly.