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.
The 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 8.
Mohammed Abu Ghama starts a small fire to heat water for tea under a destroyed tower of an apartment complex which he calls home and that was heavily damaged in fighting between Israel and Hamas. (AFP Photo)
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.
Gaza residents fatalistic
Across Gaza, the calm gave people an opportunity to return to homes they had fled.
Workmen were seen trying to clear the debris from Gaza's sole power station, which was badly damaged in the fighting two weeks ago.
As the clock ticked down to the midnight deadline, many were beginning to worry about a resumption of the violence.
"We're all worried, it's natural," said Hussein Abu Haseera, sitting outside his air conditioning shop in Gaza City's Rimal neighbourhood.
"We want this to be finished, for the blockade to be lifted. No one likes dying do they?"
In Cairo, the truce talks at the General Intelligence headquarters were expected to run until late as Egyptian mediators raced to bridge the gaps between the two sides.
The negotiations were at "a very sensitive stage," Palestinian delegation head Azzam al-Ahmed told AFP, saying he hoped they could reach an agreement before the deadline.
Few details about the substance of the talks have been made public. The teams gather in separate rooms and never see each other, with mediators shuttling between them with proposal and counterproposal, a source said.
Two boys walk past destroyed homes in the northern Gaza strip a few miles away from the border with Israel. (AFP Photo)
Unblocking the blockade
The Palestinian delegation, which includes representatives of Gaza's de facto rulers Hamas, is demanding an end to Israel's eight-year blockade of the tiny coastal enclave. Israel is understood to have refused.
Israel has said it will facilitate Gaza's reconstruction only if the enclave is fully disarmed, a demand rejected by the Palestinians.
Delegation officials said Israel had so far proposed easing restrictions at two of the six border crossings it shares with the small coastal enclave. But it had rejected a key Hamas demand for an airport and sea port in Gaza.
Israel had also proposed that militants hand over the remains of two Israeli soldiers killed in the conflict in exchange for the release of Palestinian prisoners, according to one official.
But the Palestinian delegation wants that issue to be handled separately and not as part of a truce deal.
Both sides have said they would be willing to see Palestinian president Mahmud Abbas, who heads the Western-backed Palestinian Authority, assume responsibility for the reconstruction of Gaza and be responsible for implementing any agreement reached in Cairo.
Both sides said they were ready to resume hostilities if the talks failed again.