The blast, which struck in the northern town of Beit Lahiya, killed four Palestinians and an Italian cameraman working for international news agency the Associated Press.
Gaza's interior ministry said its top bomb disposal expert in the north had been killed, naming him as Taysir Lahum.
Six people were also seriously wounded, medics said.
he deadly blast came as the warring sides observed a three-day truce which was due to end at midnight (2100 GMT).
As the clock ticked down, Israeli and Palestinian negotiators in Cairo battled to thrash out a more permanent end to the violence which began on July 8th.
The Associated Press confirmed one of its journalists had been killed, identifying him as Simone Camilli, a 35-year-old Italian cameraman who had worked for the agency since 2005.
One of its Palestinian photographers, Hatem Moussa, was also badly wounded in the explosion.
Camilli is the first foreign journalist to be killed in the violence in Gaza, which has killed more than 1,950 Palestinians and 67 on the Israeli side.
He was at the scene with the photographer to cover the story of bomb experts dismantling unexploded ordnance, the AP said.
Italian Foreign Minister Federica Mogherini expressed sorrow at Camilli's death.
"Simone Camilli's death is a tragedy for his family and for the country. Once more a reporter pays the price for a war that has lasted too many years," she said.
The deaths came as Egyptian mediators scrambled to secure the two sides' agreement to an extension of the three-day ceasefire.
Without agreement on an extension or a long-term truce, the two sides risk a resumption of the deadly fighting.
No violations of the ceasefire have been reported, but an earlier truce collapsed into new violence on Friday after Hamas refused to extend the lull, accusing Israel of stonewalling over its demands for an end to the blockade of Gaza.