Gaza's largest desalination plant will be operational in the autumn as the Palestinian enclave tries to avert a potential humanitarian crisis over a lack of drinkable water, officials said Tuesday.

Israeli aircraft hit two Hamas targets in Gaza early on Saturday in response to rocket fire as the worst flare-up of violence since a 2014 war entered a fourth day.

There was no immediate claim of responsibility for the rocket launch and most such fire since 2014 has been carried out by fringe Islamist groups but Israel holds Hamas responsible for all such attacks.

The Islamic Jihad militant group said on Monday that one of its fighters was killed and two wounded in an explosion during a training session in the northern Gaza Strip.

The armed wing of the Palestinian militant movement, the Al-Quds Brigades, said in a statement that 32-year-old Mazen Loulou was killed "while carrying out his combat duties".

The only power plant in the Hamas-run Gaza Strip was at a standstill on Tuesday due to an ongoing shortage of fuel caused largely by a dispute over taxes, officials said.

The plant has been closed since Saturday night when it ran out of fuel, said Ahmed Abu Al-Amreen, an official in the energy ministry.

A six-year-old girl wounded in an Israeli air strike on the Gaza Strip Saturday has died of her injuries, Palestinian medical sources told Agence France Presse, naming her as Israa Abu Khussa.

Her brother Yassin, 10, died earlier in a raid on a base of Hamas's military wing near their home in the northern Gaza village of Beit Lahiya. Another brother, Ayub, 13, suffered moderate injuries.

The military wing of the Palestinian Islamist movement Hamas on Friday buried one of its men killed in the collapse of a Gaza tunnel, the latest in a string of such incidents.

Ezzedine al-Qassam said its fighter Mohammed al-Astal "was martyred while working in one of the tunnels of the resistance", referring to an underground network built for fighting Israel.

Seven Hamas militants were confirmed killed Thursday after a tunnel built for fighting Israel collapsed in the Gaza Strip, highlighting concerns that yet another conflict could eventually erupt in the Palestinian enclave.

The Ezzedine Al-Qassam Brigades, Hamas's armed wing, said that four militants managed to escape after the collapse late on Tuesday in the northeast of the strip.

A military court in the Hamas-run Gaza Strip on Wednesday sentenced four Palestinians to death on charges of spying for Israel, a statement and court sources said.

The court said in a brief statement that a 23-year-old man from the Zeitoun area south of Gaza City had been sentenced to death on espionage charges. It did not provide his name.

The Israeli air force carried out attacks on Hamas sites in the Gaza Strip early Saturday, Palestinian security sources said, just hours after rockets from the enclave hit southern Israel.

According to the sources, the Israeli strikes targeted four empty facilities ranging from Beit Hanoun in the north to Rafah in the south, causing damage but no casualties.

It is a bitter Christmas for Elias Manna, who will celebrate alone for the first time because his five sons have left the Gaza Strip, the Palestinian territory hit by three wars since 2008.

"At first, they left to study there," Manna, 62, said as he stood before the large wooden cross in front of a church for Latin rite Catholics in Gaza City, explaining that his sons moved to Europe.