GAZA CITY, Gaza Strip — About 17,000 Palestinian residents of the northern Gaza Strip fled their homes on Sunday and sought safety in UN shelters, heeding warnings from the Israeli military about plans for a major offensive in the area.
The warnings were issued on the sixth day of airstrikes against Hamas that have killed more than 160 people. The fighting intensified Sunday, despite international calls for a cease-fire and growing concerns about the mounting civilian death toll in Gaza.
Early Sunday, the Israeli air force dropped leaflets around the northern Gaza town of Beit Lahiya ordering people to evacuate their homes. Israel says much of the rocket fire into Israel has come from the area.
More than 100 rockets were fired out of Gaza into Israel on Sunday, with two intercepted over the Tel Aviv area. Shortly before nightfall Sunday, Israel carried out a series of airstrikes in Beit Lahiya, with some of the heaviest bombardment of the current conflict.
Hamas’s Al-Aqsa TV station reported four airstrikes in a 10-minute span, and a large plume of black smoke could be seen over the area from the Israeli border. There were no immediate reports of casualties.
The night before, Israeli commandos carried out a brief ground operation on what it said was a rocket-launching site that could not be struck from the air. Four Israeli soldiers were slightly wounded in the operation, which was the first known deployment of Israeli ground troops in the recent round of fighting.
With Israel massing tanks and thousands of soldiers at Gaza’s borders, some fear that could signal a wider ground offensive.
The UN refugee agency for Palestinians, UNRWA, said 17,000 Palestinians had headed to special shelters set up in 20 United Nations schools in Gaza, mostly from areas around Beit Lahiya and Beit Hanoun.
Sami Mshasha, an agency spokesman, said the hasty evacuations were an indication of the “difficult situation on the ground.’’
‘‘Once we received the message, we felt scared to stay in our homes. We want to leave,’’ said one resident, Mohammed Abu Halemah.
As diplomacy continued, Israel said it was planning to increase bombardment and push forward with preparations for a possible ground invasion.
‘‘We don’t know when the operation will end,’’ Prime Minister Benjamin Netanyahu of Israel told his Cabinet on Sunday. ‘‘It might take a long time.’’ He said the military was prepared ‘‘for all possibilities.’’
Secretary of State John Kerry spoke Sunday to Netanyahu and said the United States is ready to help restore calm, while Egypt, a key mediator between Israel and Hamas, continued to work behind the scenes.
UN Secretary General Ban Ki-moon called for an immediate cease-fire in a statement issued late Sunday.
“It is in the interest of both sides that steps toward dangerous escalation be replaced with immediate measures to end the fighting, thus preventing further casualties and greater risks to regional peace and security,’’ the statement said.
Ban condemned Hamas’s indiscriminate firing of rockets against Israeli civilian targets as ‘‘a violation of international law,’’ it said. At the same time, the UN chief is ‘‘deeply worried about the impact on Palestinian families of Israeli military action.
Israel launched the air offensive last Tuesday in what it said was a response to heavy rocket fire from Hamas-controlled Gaza. The military says it has launched more than 1,300 airstrikes, while Palestinian militants have launched about 800 rockets at Israel.
The Palestinian Health Ministry in Gaza says 166 people have been killed, including dozens of civilians. There have been no Israeli fatalities, though several people have been wounded, including a teenage boy who was seriously injured by rocket shrapnel Sunday.
Hamas, an Islamic militant group sworn to Israel’s destruction, has remained defiant, and it continued to fire rockets into Israel throughout the day. It urged people in northern Gaza to stay in their homes and has so far rejected proposals for a cease-fire as unsatisfactory.
‘‘They want us to put down our arms and leave the resistance,’’ said Moussa Abu Marzouk, a top Hamas official, on his Facebook page. ‘‘They started the battle, and we will stay on our land and fight to protect our future.’’
Despite Israeli assertions that it has inflicted heavy damage on the group, Hamas says it is largely unscathed, and Palestinian medics say most of the dead have been civilians.
The outbreak of violence follows the kidnappings and killings of three Israeli teenagers in the West Bank, the kidnapping and killing of a Palestinian teenager in an apparent revenge attack, and wide-ranging Israeli moves against Hamas militants and infrastructure in the West Bank.
Hamas has demanded that hundreds of recently arrested activists be freed as part of a cease-fire.
Many of the airstrikes have been on the homes of wanted Hamas militants, putting their families at risk. In an attack on Saturday, the target of one such airstrike, Gaza’s police chief, survived while 17 members of his extended family were killed.
Israel accuses Hamas of using Gaza’s civilians as human shields, putting people in the densely populated territory in danger.