Israeli forces backed by artillery barrages and airstrikes launched a major ground offensive into Gaza late Thursday, marking a dramatic escalation of the 10-day-old conflict with the Palestinian militant group Hamas.

Spokesmen for the Israel Defense Forces said the goal of the offensive was limited: to destroy tunnels used by the militants to infiltrate into Israel. “This requires precise operations,” their statement said. Among the troops moving in were engineering units with armored bulldozers.

“We have hit Hamas hard and we will continue to hit Hamas hard,” Israel’s military said in a tweet shortly after thousands of Israeli soldiers entered Gaza.

It was Israel’s first ground incursion into Gaza since January 2009, when it engaged in a three-week battle with Hamas that killed more than 1,400 Palestinians and 13 Israelis. Thursday’s operation came hours after diplomatic efforts in Cairo to negotiate a long-term truce between Israel and Hamas failed.

The assault creates significant challenges and risks for Israel. Casualties could soar in a conflict that, as of Thursday, had claimed more than 200 Palestinian lives and left one Israeli dead. Israeli soldiers also face the prospect of a guerrilla war in Gaza’s densely populated territory.

Hamas has bolstered its arsenal since seizing control of Gaza in 2007. Over the past 10 days, it has shown a capability to target Israeli towns with longer-range rockets even as it faces heavy air bombardments by Israel. On Thursday evening, Hamas spokesman Fawzi Barhom warned that the Israelis “have taken a dangerous step.”

“They did not study the consequences,” he added. “The occupation forces will pay a high price, and Hamas is ready for this confrontation.”

The ground offensive began at 10 p.m. local time and involved dozens of infantry and artillery units, as well as air and naval support, said an Israel Defense Forces spokesman, Lt. Col. Peter Lerner. The military, he added, plans to call up 18,000 reserve troops in addition to the 50,000 reservists activated so far.

Lerner said the military’s activities along the southern part of the Gaza Strip were carried out in coordination with Egypt but did not elaborate.

In Gaza City, from a high floor in an apartment building, reporters watched heavy fire along the eastern and northern borders of the Gaza Strip.

Heavy firepower

The sky was lit as if by lightning strikes. There were sustained barrages of artillery, flares shooting through the night and drones flying overhead. Israeli warships fired from the Mediterranean Sea.

Hamas’s military wing, the Al-Qassam Brigades, reported fighting with Israeli troops near the Erez crossing, the only passage for people traveling between Israel and Gaza.

Electricity was out across Gaza City, and local residents poured into the Al Deira Hotel, where many foreign journalists are staying.

Ambulance drivers could be heard on police scanners saying they could not reach neighborhoods because of the shelling. Families who remained behind in areas that the Israelis warned should be cleared were calling relatives in Gaza City to say that the bombardment had trapped them.

It was unclear what triggered the ground invasion, although it had been under discussion in Israeli political and military circles for days. One possibility was an attempt by Hamas militants to infiltrate into Israel early Thursday morning.

The Israeli air force hit the opening of a tunnel used by the militants to sneak under the Gaza border fence. The opening was about 250 yards inside Israel and a little more than a mile from a kibbutz.

The ground assault came hours after a short break in the fighting. Residents rushed outside their homes to take advantage of a five-hour “humanitarian truce” between Israel and Hamas requested by the United Nations. It was a brief respite on a day that began with mortar fire from Gaza and ended with the ground assault.

During the lull, a group of men at a mosque in northern Gaza said they had returned to clean up the green glass from windows shattered in the previous day’s bombardment. But they could be seen moving small rockets into the mosque.

“We want a cease-fire, but not a cease-fire at any cost. We’re not animals,” said Moufeed al-Hasainah, the Hamas housing minister, who was visiting a hospital in Gaza City. “What is a cease-fire without an open border crossing? Without salaries? Without jobs?” he said.

Khalid al-Batish, a political leader of Islamic Jihad, another militant group, said the armed factions were prepared to keep fighting until their cease-fire conditions were met.

“A truce should include a reward for the Palestinian people,” he said.

Occupation called unlikely

Despite their determination to destroy Hamas’s military infrastructure, Israeli military officials have said in recent days that they don’t seek to reoccupy Gaza, seeing that as creating a host of new problems. Nor are they interested in overthrowing the Hamas government in Gaza, fearing that a power vacuum could result, with dozens of militant factions vying for control.

Analysts expect the Israeli ground forces to focus on destroying the tunnels and trying to avoid engaging Hamas fighters as much as possible.

“If all that will stop the shooting from Gaza into Israel, I believe that after a few days they will go back [home],” said Itamar Yaar, a former top official at Israel’s National Security Council. But he added that if Hamas attacks Israeli soldiers, the “Israeli cabinet will not have any other choice other than to give orders to the armed forces to go deeper.”

Israel’s housing minister, Uri Ariel, a hard-liner, said in an interview with Israel’s Channel 2 News that Israeli troops had to “finish the work.”

“We will not reach a cease-fire in the next few days; it will take longer,” he said. “I believe it will not end with just destroying the tunnels.”

Just as the temporary cease-fire ended at 3 p.m. local time Thursday, Palestinian militants in the coastal enclave fired a rocket at the Israeli city of Ashkelon, about eight miles north of the Gaza border, but no injuries were reported, the Israeli military said. Three mortar rounds were also fired toward Israel after the truce took effect at 10 a.m.

Lerner said at least one militant was believed to have been killed in the early-morning strike on the tunnel and that the remaining fighters apparently returned to Gaza. He said the militants were armed with “extensive weapons,” including rocket-propelled grenades, the Associated Press reported.

The al-Qassam Brigades claimed responsibility for the infiltration, asserting that the airstrike came after the fighters had successfully completed their mission. Hamas denied that any of its members were killed.

“All our mujahideen came back safely,” the group said in a statement.

Raghavan and Eglash reported from Jerusalem. Karen DeYoung in Washington and Islam Abdel Karim in Gaza City contributed to this report.