The Al-Quds Brigades, the armed wing of Islamic Jihad, said Raid Faanuna was killed in an air strike "during a mission" east of Gaza City.
"The refusal by the US, EU et al to deal with Hamas reflects their disrespect for the wishes of the Palestinian people"
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The Israeli military has denied carrying out any air strikes.
Witnesses said they saw an Apache helicopter flying low and heard heavy shooting and periodic explosions as Israeli soldiers patrolled rooftops and Palestinian fighters ran through the streets.
Diaa Abu Dhakar, another Al-Quds Brigades fighter, was killed during clashes with Israeli soldiers in Khan Yunis - where seven more Palestinians were wounded as clashes flared
Twenty Palestinians, including seven in critical condition, were taken to hospital in Gaza City.
In the south, two fighters form the Al-Quds Brigades and Hamas's armed Izz al-Din al-Qassam Brigades were killed in the fighting. Another two Palestinians were wounded.