The spokesman for the Israeli Defense Forces, General Daniel Hagari, pointed out that the offensive is taking place at the brigade level and is backed by the aviation.
Hagari declined to set a timetable for the campaign but hinted that it would last a minimum of several days and possibly even weeks or months.
The Palestinian Ministry of Health has so far reported four deaths and 27 wounded, seven of them seriously injured.
Israeli press reports indicate that the IDF occupied key areas in Jenin and that it might start raiding homes in the town in search of militants in the next few hours.
The WAFA news agency reported that a large contingent of soldiers and some 150 military vehicles, including bulldozers, entered Jenin and its camp from various directions after an initial bombardment using drones. It added that the Israeli army cut off the roads linking the two towns and deployed numerous snipers on rooftops.
The Arutz Sheva news website reported that the government of Benjamin Netanyahu threatened a harsh response against the Gaza Strip if the Islamic Resistance Movement (Hamas) or the Islamic Jihad fired missiles in retaliation.
The leader of the Israeli opposition and former prime minister, Yair Lapid, praised the operation, as did other critical leaders of the Executive, including the former head of Defense Benny Gantz.
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