Israeli Prime Minister Benjamin Netanyahu on Thursday instructed the military to begin a ground offensive in Gaza, an official statement from his office said.
"The prime minister and defense minister have instructed the IDF to begin a ground operation tonight in order to hit the terror tunnels from Gaza into Israel," the statement said.
Reuters witnesses and Gaza residents reported heavy artillery and naval shelling along the Gaza border.
Fierce fighting between Israel and Hamas resumed Thursday, including an airstrike that killed three Palestinian children feeding pigeons on their roof, after a temporary cease-fire that allowed Gazans to stock up on supplies.
The resumption of violence signaled major obstacles to Egyptian-led efforts to reach a permanent truce between the two sides, despite increased efforts by Egypt to broker a deal.
The U.N.-brokered five-hour lull in fighting gave residents of the Gaza Strip time to crowd into stores and vegetable markets after more than a week of being mostly holed up at home for fear of airstrikes.
But the streets emptied out quickly after the cease-fire expired, with Palestinian militants firing more than 50 rockets at Israel, including a heavy salvo toward the Tel Aviv area that sent people running for cover, the Israeli military said.
Israel responded with a wave of eight airstrikes, including one that killed two boys and a girl ages 8 to 10 from the same family in Gaza City, Gaza Health Ministry spokesman Ashraf al-Kidra said.
Their grandfather, Marzouk Shahaibar, said the cousins had gone on the roof of the home to feed pigeons.
"Without even realizing it, they were struck from above," he said tearfully. "They weren't fighting. They did nothing."
The deaths came a day after four boys ages 9 to 11 were killed on the beach beside a coastal road west of Gaza City. Israel issued a renewed warning Thursday to Gaza residents to leave their homes for their own safety.
Another Israeli airstrike hit a crowded area outside a mosque and shattered windows in a nearby care center for the disabled and elderly, Palestinian security officials said.
The director of the Wafa Rehabilitation Center, Basman Ashi, said four staff members were hurt and power went off in the building after the attack.
The center in the Gaza Strip has 17 patients, some on respirators or unable to walk.
The Israeli military has repeatedly warned Ashi to evacuate his patients ahead of impending strikes, saying the area near the Israeli border is being used by Hamas to launch rockets.
Ashi has refused, saying he can't move the patients safely. Foreign volunteers have stayed with the patients to try to deter the strikes.
The Israeli army had no immediate comment.
Mounting civilian casualties have increased international pressure to stop the hostilities, but negotiators have made little headway.
Egyptian efforts to bring all sides to the table continued Thursday, with President Abdel Fattah el-Sissi meeting with Palestinian leader Mahmoud Abbas after Egyptian officials met separately with representatives of Israel and Hamas in Cairo. But the gaps remain wide.
Israel accepted Egypt's call earlier this week to halt all fighting, but Hamas rejected the idea because it first wants to lock in achievements, such as easing the seven-year blockade of Gaza by Israel and Egypt.
Strict Egyptian access restrictions to Gaza over the past year, combined with long-running Israeli closures, severely weakened the Islamic militant group.
During the five-hour humanitarian cease-fire, Gaza's streets were jammed with traffic, motorists honking horns and Hamas police directing traffic at busy intersections.
Hundreds lined up outside banks, with people jostling and shouting to get to ATM machines. In an outdoor market, shoppers filled plastic bags with fruit, vegetables and freshly slaughtered chickens.
"The situation is likely to get worse because there is no clear way out of it," said Moussa Amran, 43, a money changer in central Gaza City.
Israel has carried out nearly 2,000 airstrikes and Hamas has fired more than 1,300 rockets since the current conflict erupted on July 8 as anger spread over the killings of three Israeli teenagers followed by the burning death of a Palestinian teen in an apparent revenge attack.
In Jerusalem, a 29-year-old Jewish man and two teenagers were indicted Thursday on charges of murder and kidnapping in Mohammed Abu Khdeir's death.
The indictment said that the suspects — who have not been identified — carried out the crime to avenge the deaths of the three Israeli teens last month in the West Bank and that they killed Abu Khdeir "solely because he was an Arab." The suspects also were accused of attempting to kidnap a 7-year-old Arab boy a day earlier.
Abu Khdeir was strangled, beaten and burned to death while he was unconscious, according to the indictment.
On Thursday, Israel's Ministry of Defense recognized Abu Khdeir as a "victim of terrorism."
Israeli airstrikes have killed more than 235 Palestinians have been killed, according to the Gaza Health Ministry. One Israeli has been killed and several wounded in rocket attacks, officials said.
Many of the rockets fired from Gaza have reached beyond the border area to Israel's economic and cultural heartland, but the Israeli casualty toll has been kept low due to the success of its "Iron Dome" missile defense system, which has shot down more than 300 incoming rockets. The military said Thursday that the system has shot down 86 percent of its targets since the current round of fighting began.
Israel accuses Hamas of firing from within populated neighborhoods, using civilians as "human shields" and mosques, homes and schools for storing weapons.
On Thursday, the U.N. refugee agency for Palestinians, UNRWA, said that during a routine check it discovered about 20 rockets hidden in one of its vacant Gaza schools and called on militants to respect the "sanctity and integrity" of U.N. property. It said the incident was "the first of its kind in Gaza."
In Cairo, Egypt's Foreign Minister Sameh Shukri insisted in an interview with The Associated Press that the cease-fire deal was still alive and expressed frustration that "Palestinian factions" — a clear reference to Hamas — had not agreed to it.
Hamas' agreement is crucial to any such truce, but its demand that the blockade be eased significantly is likely to be rejected by Israel and Egypt because it would strengthen the group's hold on Gaza, where it seized power in 2007.
On Wednesday, the No. 2 in Hamas, Moussa Abu Marzouk, presented a list of demands to Egypt, including that Gaza's crossings be opened and all types of goods be allowed into the territory, said a senior official in the group who spoke on condition of anonymity because he was not authorized to discuss the negotiations with reporters.
Israel has allowed consumer goods into Gaza, but has restricted construction material, fearing it would be diverted for military use. Israel has also barred most exports from Gaza, crippling the local economy.
Hamas also wants to be allowed to build a sea port as a gate to the world, with shipments under international monitoring, the Hamas official said.
In addition, Hamas demands the release of 52 activists who had been released by Israel in a 2011 prisoner swap, but were rearrested in recent months.
In the lead-up to Thursday's lull, 13 heavily armed Hamas militants tried to sneak into Israel through a tunnel from Gaza but were struck by Israeli aircraft at the mouth of the tunnel some 250 meters (820 feet) inside Israel, near a kibbutz, the military said.
Lt. Col. Peter Lerner, a military spokesman, said the military believed at least one militant was killed in the strike and the remaining fighters appeared to have returned to Gaza through the tunnel. Hamas' military wing said all its fighters returned safely.
Israeli aircraft also struck 37 other targets earlier Thursday before the temporary truce took effect, including the homes of senior Hamas leaders Fathi Hamad and Khalil al-Haya, the military said.