Israel on Tuesday unleashed its heaviest bombardment in a three-week-old war against Hamas, striking symbols of the militant group's control in Gaza and firing tank shells that Palestinian officials said shut down the strip's only power plant.

Thick black smoke from the plant's burning fuel tank rose for hours. The station's shutdown further disrupted the supply of electricity and water to the 1.7 million people packed into the narrow Palestinian coastal territory.

Israel's Foreign Ministry spokesman, Yigal Palmor, denied that Israel shelled the power plant in an interview with CBC's Power and Politics. While Palmor said he didn't know how the plant was damaged, he said "it wasn't anything that had to do with Israeli army operations."

On Tuesday evening, residents of the sprawling Jebaliya refugee camp in northern Gaza reported intense tank shelling. Ten members of an extended family were killed and 50 other people wounded in the area, Palestinian health officials said. Two brothers driving in a car with markings of a UN aid agency were killed by shrapnel, an area resident said."

A Palestinian firefighter tries to put out a fire at Gaza's main power plant, which witnesses said was hit Tuesday in Israeli shelling. Witnesses said the fuel storage at the power plant was struck. (Ahmed Zakot/Reuters)

"It was like an earthquake," Moussa al-Mabhouh, a volunteer for Gaza's Civil Defense, said of the scene. "Roofs collapsed, walls cracked and wounded people everywhere."

The heavy strikes — which came a day after Prime Minister Benjamin Netanyahu on Monday warned of a "prolonged" campaign against Hamas — were a new blow to international efforts to reach a sustainable truce in the fighting.

Israel has vowed to stop the Hamas rocket and mortar fire that has reached increasingly deeper into its territory and to destroy a sophisticated network of tunnels that have been used by the militants to infiltrate the Jewish state. For its part, Hamas has so far rejected ceasefire efforts unless its demands are met, including a lifting of a punishing blockade.

At least 1,175 Palestinians have been killed since the start of fighting July 8, including at least 100 on Tuesday, said Palestinian health official Ashraf al-Kidra.

Israel has reported 53 soldiers and three civilians killed.

Gaza to brace for power crisis

The scene at the Gaza power plant after two tank shells hit one of three fuel tanks was daunting. "We need at least one year to repair the power plant, the turbines, the fuel tanks and the control room," said Fathi Sheik Khalil of the Gaza Energy Authority. "Everything was burned."

Smoke rises from Gaza port after a series of explosions in Gaza City July 29. (Finbarr O'Reilly/Reuters)

He said crew members who had been trapped by the fire for several hours were evacuated.

Even before the shutdown, Gaza residents only had electricity for about three hours a day because fighting had damaged power lines. Most of the power lines from Israel that provided electricity for payment were previously damaged in the fighting.

This means most of Gaza will now be without power. The lack of electricity will also affect water supplies, since power is needed to operate water pumps.

Rockets found at Gaza school: UN

The UN agency that looks after Palestinian refugees said it had found a cache of rockets at one of its schools in the Gaza Strip and deplored those who had put them there.

United Nations Relief and Works Agency (UNRWA) spokesman Chris Gunness condemned those responsible for placing civilians in harm's way by storing the rockets at the school but he did not specifically blame any particular party.

Israel has targeted a few UNRWA sites during fighting in the current 22-day-old campaign against Islamist militants in the Gaza Strip, and has in the past said the agency's property was used for hostile purposes.

Gunness said the body had called in a U.N munitions expert to dispose of the rockets and make the school premises safe, and added that he could not get to the site due to fighting in the area.