They said the Israeli forces crossed the border in the area of the town of Beit Hanoun, in the northeastern corner of Gaza, opposite the Israeli town of Sderot, a frequent target of Palestinian rockets. The Israeli vehicles were positioned in Palestinian farmland outside the town, the residents said, and were approaching populated areas.
The Israeli military would not officially confirm the new incursion.
Witnesses and Palestinian security said the new operation started with machine gun and shell fire, then about 10 tanks and bulldozers crossed into northern Gaza, with dozens of other armored vehicles poised just inside Israel.
Ten days ago Israeli forces carried out a two-day sweep of northern Gaza, entering Beit Hanoun and another town, Beit Lahiya, looking for militants.
Israeli aircraftin other parts of the Gaza Strip earlier on Saturday, killing two people, officials said.
The first attack hit offices of the Hamas-controlled Economy Ministry in Gaza City, setting the building on fire. Rescue teams rushed to the scene of the attack, located near a housing complex inhabited by about 600 people.
An Israeli airstrike then destroyed a small bridge near the Bureij refugee camp in central Gaza, the army said.
A separate airstrike on a Hamas rocket workshop in Gaza City killed one Palestinian and wounded seven people, Palestinian medics said. The medics had initially said two people were killed, but one turned out to be seriously wounded. The building was demolished in the attack and Palestinian rescue workers were searching through the rubble.
Israeli helicopters also opened fire on an area in northern Gaza where militants had gathered to fire rockets at southern Israel, killing an 18-year-old Bedouin, Khalil Abu Naghafieb, said hospital officials.
Israel started an offensive in Gaza on June 28, three days after Hamas-backed militants tunneled under the border and attacked an Israeli army post, killing two soldiers and capturing a third. By Friday, Israeli forces had pulled out of the three sections of Gaza where they had been operating.