Day 13 was the bloodiest so far. More than 100 Palestinians were killed in heavy bombardment and street battles in Gaza on Sunday and 13 Israeli soldiers were slain in the most intense day of fighting in Israel’s current offensive against Hamas, officials said.

Israel said it pummeled a neighborhood in east Gaza because the warren of shops and concrete-block homes was the site of frequent rocket launches and concealed a network of tunnels dug by Hamas fighters and allied militant factions.

When their troops went in, Israeli military officers say, they were surprised by the tenacity, training and weaponry of their opponents. They said Israeli soldiers were repeatedly hit by Gaza militants firing from windows, employing land mines and setting booby traps.

“It was a very hard battle there,” a senior Israeli military official said. “I have to admit that we were facing good fighters from the other side.”

The seven-hour attack by Israeli artillery and tank shells, followed by small-arms gun battles in the streets, left the district in ruins. There were bodies in the streets and gray-faced Palestinians being dug out of the rubble and stacked into ambulances. Thousands of residents had fled in the middle of the night, many barefoot.

The Gaza Health Ministry said 70 Palestinians were killed in the fighting in the Shijaiyah district on the eastern outskirts of Gaza City, the new front line. In all, more than 100 Palestinians died Sunday. Hamas health officials, in keeping with their practice, did not say whether the dead were civilians or fighters.

Sami Abu Zohri, a Hamas spokesman, called the Israeli offensive in Shijaiyah “a massacre” and “a war crime.”

More than 445 Palestinians, many of them women and children, have died in the nearly two-week-long offensive.

The Hamas military also asserted that its fighters had captured an Israeli soldier. Abu Obaida, a spokesman for the Izzedine al-Qassam Brigades, appeared on Hamas TV to make the claim. Minutes later, there were fireworks on the streets and shouts of “God is great!” from loudspeakers in the mosques.

An Israeli military spokesman said the army was investigating and did not confirm the abduction. “There's no kidnapped Israeli soldier and those rumors are untrue,” Ron Prosor, Israel’s U.N. ambassador, told reporters in New York.

It was the single worst day so far for Israelis. Israeli military deaths have risen to 18 since the armed forces launched a ground operation Thursday.

The State Department late Sunday confirmed the deaths of two U.S. citizens in Gaza, Max Steinberg and Sean Carmeli. “Out of respect for those affected by this, we have nothing further at this time,” spokeswoman Jen Psaki said in a statement.

In a statement early Monday, The Jewish Federations of North America said Steinberg was from Woodland Hills, Calif., and Carmeli from South Padre Island, Tex.

The Israeli death toll is now higher than during the Israeli military’s 2009 incursion into Gaza, when 13 Israelis were killed.

More than 77 soldiers have entered hospitals countrywide, some of them seriously wounded.

President Obama, in a phone call with Israeli Prime Minister Benjamin Netanyahu, “raised serious concern about the growing number of casualties,” according to the White House. Secretary of State John F. Kerry’s office announced he would fly to Cairo on Monday, seeking international help in brokering a cease-fire.

As the U.N. Security Council gathered Sunday night for an emergency meeting requested by council member Jordan, Secretary General Ban Ki-moon called Israel’s Shejaiyah operation an “atrocious action,” and said that “Israel must exercise maximum restraint and do far more to protect civilians.”

Ban, who spoke in Qatar, traveled to the region Saturday on what he said was a “journey of solidarity and peace.” He plans to visit Cairo as well as Jerusalem and Ramallah, on the West Bank. He called on all sides to “respect international law.”

A Jordan-drafted council resolution expresses “grave concern” about the high number of civilians killed in Gaza, including children, and it calls for an immediate cease-fire, “including the withdrawal of Israeli occupying forces from the Gaza Strip,” the Associated Press reported.

‘We are not deterred’

Seven Israeli troops from the Golani Brigade were killed in the streets of Shijaiyah by Gaza militants who detonated an antitank mine beneath their armored vehicle. Three soldiers were fatally shot in ambushes and firefights, and three died trapped in a burning building, military spokesmen said.

“The heart of the entire nation is with the families of the fallen,” Netanyahu said at a news briefing at the Israeli defense headquarters in Tel Aviv, held before news of the alleged capture broke.

“We feel your pain. We bow our head to your sons who fell so that we can continue living here,” the prime minister said. “We are not deterred, and we will continue to act as necessary. The current operation is necessary for the security of Israel’s citizens.”

When soldiers of the Golani Brigade entered Shijaiyah, Hamas militants were prepared to ambush them, the senior Israeli military official said in a briefing with foreign correspondents Sunday night.

The Israeli military had warned residents a few days earlier of the impending push into the neighborhood and told them to evacuate. The militants used that information to their advantage.

“They knew exactly when and where to wait for us,” said the official, who spoke on the condition of anonymity because of military protocol.

Hamas militants, he added, were better fighters than the Israelis had anticipated. They were “highly trained” and “very well equipped” with sophisticated weapons systems. They accurately fired light and heavy weapons, including mortars, at the Israeli forces while “setting up a lot of booby traps,” he said.

The fighting in Shijaiyah lasted for more than seven hours, the official said. Many civilians had remained in the enclave because Hamas fighters had ordered them not to leave, effectively employing them as human shields, the Israeli official said.

Residents, however, said they had been told to move before by the Israelis, only to return to their homes. Some said they were reluctant to leave their homes just because the hated Israelis told them to.

Brief cease-fire

Sunday morning, the streets of Gaza City filled with thousands of panicked residents pouring out of Shijaiyah during a lull in the battle. Many sought shelter at Shifa Hospital, Gaza’s main medical center, where the emergency rooms were overflowing and surgeons were treating patients in the corridors.

The Red Crescent humanitarian organization announced it had negotiated a two-hour cease-fire in the afternoon that allowed ambulances access to Shijaiyah. In 10 minutes, on one street, seven stretchers were carried out of the rubble. Some of the injured looked near death.

Also darting from house to house were militants dressed in black, their faces covered with kaffiyeh scarves, carrying Kalashnikov rifles. One trio ran past a group of reporters and entered a house down the lane.

Israeli Economy Minister Naftali Bennett, one of the coalition government’s hard-liners and a former company commander of special forces, said, “I do not rule out the goal of toppling the Hamas regime” that governs Gaza.

“There is a world of weapons tunnels penetrating into Israel, creating the possibility of a mega-attack,” Bennett said.

Capt. Eytan Buchman, an Israeli military spokesman, said the armed forces had no intention of reoccupying Gaza, which they pulled out of in 2005. He said the main objective of the ground offensive hadn’t changed: to destroy the tunnel network and rocket launchers that Hamas militants have used to attack Israel.

“We’ve expanded the forces on the ground in order to accomplish that mission,” Buchman said. “All of Gaza is an underground city, and the amount of infrastructure Hamas built up over the years is immense. There are tunnels, extended bunkers, weapons storage facilities, even within urban areas.”

The Israeli military said Shijaiyah had also been targeted because 8 percent of all rockets fired by Hamas and other militant groups into Israel since the conflict began have come from the town.

Obama and Netanyahu spoke on the telephone Sunday, their second call in three days to discuss Gaza.

According to the White House, Obama “reiterated the United States’ condemnation of attacks by Hamas against Israel, and reaffirmed Israel’s right to defend itself.”

Raghavan reported from Tel Aviv. Eglash reported from Kfar Aza, Israel. Islam Abdel Karim in Gaza and Karen DeYoung in Washington contributed to this report.