Israel Orders ‘Complete Siege’ of Gaza and Mobilizes 300,000 Reservists

Israel ordered a “complete siege” of the Gaza Strip on Monday as it retaliated for the largest and deadliest incursion into its territory in decades, bombing hundreds of sites in Gaza, including mosques and a marketplace, and battling Palestinian fighters to regain control of Israeli towns near the border.

Defense Minister Yoav Gallant of Israel said that “no electricity, no food, no water, no fuel” would be allowed into Gaza, the crowded and impoverished coastal territory that is already under a 16-year blockade by Israel and Egypt, and is controlled by the militant group Hamas. Israel mobilized 300,000 military reservists, an enormous number for a country of 9 million people, amid signs that it could be preparing for a major ground invasion of Gaza and the possible opening of another front against the Lebanese Shiite organization in the north.

At least 800 people have been killed in Israel and more than 2,600 wounded since the incursion began early Saturday, and Hamas gunmen were holding 150 hostages they seized in Israel, the Israeli government said. The spokesman for Hamas’s military wing, Abu Obeida, threatened that it would execute a civilian hostage every time an airstrike hit “our civilians in their homes without warning.”

At least 560 Palestinians were killed and at least 2,900 injured in Israeli airstrikes, the authorities in Gaza said.

Israel has dispatched tanks and troops to its southwestern territory bordering Gaza, and Prime Minister Benjamin Netanyahu warned on Sunday of “a long and difficult war that was forced on us by a murderous Hamas attack,” adding that Israel was going on the offensive, “which will continue with neither limitations nor respite until the objectives are achieved.”

Lt. Col. Richard Hecht of the Israel Defense Forces told reporters on Monday that the next phase of fighting would not resemble recent conflicts with Gaza, in which Palestinian groups fired rockets but claimed relatively few casualties, and Israel would respond primarily with airstrikes. “We are in a different game here,” he said. “We are at war with Hamas.”

A stunned disbelief enveloped Israel, which appeared to have been caught entirely by surprise by the attack and had gone generations without enduring an assault on this scale or with so many casualties. Adding to the shock were the mass hostage-takings and the fact that fighting on Israeli soil continued into a third day. Families were watching men and women who had finished their main military service being called back to duty, while the names of the dead scrolled across television screens.

An Israeli airstrike on Monday devastated a busy open-air marketplace in northern Gaza’s Jabaliya refugee camp, where Gazans anticipating a long fight had flocked to stock up on food and other supplies. A Red Crescent paramedic, who was not authorized to speak to the media and requested anonymity, said 60 people were killed there.

Videos shared on social media and distributed by Palestinian news agencies showed bodies strewn amid the debris of what moments earlier had been stands selling produce and other goods. Broken concrete and twisted metal from the surrounding buildings filled the square, and people made their way through the debris, smoke and dust, looking for survivors.

“Is he dead? Is he dead?” a man was heard yelling in one video.

Israeli strikes also hit four mosques in the Shati refugee camp on Monday, according to Gaza authorities, toppling their domes and killing worshipers inside. Neighbors picked through the rubble of the Sousi mosque, where witnesses said boys had been playing soccer just outside when it was destroyed.

Israel’s military said it had carried out more than 500 airstrikes overnight, targeting operations centers of Hamas and another group, Islamic Jihad. It confirmed hitting several mosques, saying that they contained Hamas infrastructure or fighters.

The United Nations and Palestinian officials said that at least two hospitals and multiple homes had also been hit, and many Gazans said they had nowhere to go to escape the Israeli strikes.

A White House official said nine Americans were confirmed killed in the weekend attack by Hamas and that others were missing, though it did not say if any of them were among the hostages.

“There are U.S. citizens unaccounted for,” a National Security Council spokesperson said. “We are closely monitoring information about hostages taken by Hamas.”

Many other foreign nationals were reported dead, wounded or missing, including 12 Thais killed, 11 kidnapped and nine wounded, according to their government.

At least 109 of the people killed in Israel were attending a weekend music festival at a venue three miles from the Gaza border when gunmen swarmed into the site on Saturday. Videos show panicked concertgoers fleeing south into the desert and more than 100 abandoned vehicles on the side of the road.

The concert, billed as a “psy trance music festival,” was attended by about 3,500 people near the town of Re’im. The organizer, Nimrod Arnin, said the first sign of the attack was a rocket barrage around 6:30 a.m., prompting an evacuation that began before the gunmen appeared.

Hamas’s armed wing, the Al Qassam Brigades, said four Israelis were killed in one of the Israeli strikes, along with the Palestinian gunmen who were holding them captive, a claim that could not be independently verified.

A new barrage of rockets fired into Israel from Gaza injured seven people on Monday, officials said, while sirens blared in Jerusalem and across central Israel. Schools remained closed and flights in and out of the country were curtailed.

Fighting broke out on another front, on Israel’s northern border with Lebanon, with Islamic Jihad claiming responsibility for attacks there.

It was not clear what prompted the timing of the most ambitious attack Hamas has ever launched on Israel. Hamas is backed by Iran, as is Islamic Jihad, and Tehran is eager to derail a possible diplomatic deal between its two regional archenemies, Israel and Saudi Arabia. The Iranian foreign ministry denied that its government had any role in the fighting.

Israel’s chief military spokesman, Rear Adm. Daniel Hagari, declared on Monday that the army had regained control of border communities, though “there may still be terrorists in the area.” But a short time later the military said soldiers and armed militants were exchanging gunfire in Kfar Azza, an Israeli village near the border.

Colonel Hecht said in a news briefing on Monday that Israeli special forces troops were trying to dislodge militants from a civilian area. He also said that all the breaches in the border fence had not yet been fully closed, and that more gunmen could still be crossing into Israeli territory from Gaza.

“We thought by this morning we’d be in a better place,” he said.

Mr. Netanyahu warned local leaders in southern Israel to prepare for a long fight, his office said.

“I know you have been through tough and terrible things. What Hamas will go through will be tough and terrible,” he told them, according to his office. “We are already in the midst of a battle that has only just begun.”

The United Nations humanitarian agency said Israeli strikes had displaced 123,000 Gazans, and damaged water, sanitation and hygiene facilitates affecting more than 400,000 people. The only power plant providing electricity to Gaza could run out of fuel in a few days, aid agencies have said.

Islamic Jihad’s military wing, the Al Quds Brigades, has claimed responsibility for a cross-border attack from Lebanon into Israel. The group said several Israeli soldiers were wounded in the assault, which the Israeli military has not verified.

In the north, Israel’s Home Front Command instructed the residents of 28 towns and villages near the border with Lebanon to go to bomb shelters and other protected spaces. The residents were told to take food, water, mattresses, and blankets, signaling that they may need to stay there for a while.

The Lebanese Army said Israeli planes and artillery struck near the towns of Dhayra and Aita al Shaab close to the border with Israel earlier on Monday. The military instructed Lebanese civilians “not to go to areas adjacent to the border for the sake of their safety.”

Israeli leaders are concerned that Hezbollah, the powerful Lebanese militia allied with Iran, could enter the fight, and Israeli military units in the north are on high alert.

Isabel Kershner reported from Jerusalem, Aaron Boxerman from London and Hiba Yazbek from Nazareth, Israel. Reporting was contributed by Raja Abdulrahim, Ameera Harouda, Edward Wong, Farnaz Fassihi, Cassandra Vinograd, Ronen Bergman, Sui-Lee Wee and Tiffany May.