They say the militants disabled the truck with a roadside bomb detonated remotely then opened fire.
A total of 10 conscripts were wounded in the attack on the outskirts of el-Arish, Sinai's largest city on the Mediterranean coast.
The officials spoke on condition of anonymity because they were not authorized to brief the media.
Security forces have been battling militants in northern Sinai for years in an insurgency that gathered steam after the 2013 ouster by the military of an Islamist president whose one-year rule proved divisive.
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