A sprawling network of Hamas tunnels under the Gaza Strip has become a primary target for the Israeli military in its stated mission to defeat the Palestinian militants, experts say.
Since Hamas gunmen stormed into southern Israel on October 7, launching raids that Israel says have killed 1,400 people, the military has stepped up urban warfare training at a desert base where a replica Gaza town had been built.
Israeli authorities report over 220 people were abducted from the Middle Eastern nation and are feared to be held in the tunnels.
Gaza's Hamas-run health ministry says Israeli airstrikes have killed more than 7,700 people, including 3,500 children, in three weeks of the ongoing war.
Hamas' military wing, the al-Qassam brigades, early Saturday reported clashes with Israeli troops in Gaza's northeastern town of Beit Hanoun and in the central area of Al-Bureij.
"Al-Qassam Brigades and all Palestinian resistance forces are fully prepared to confront the aggression with full force and thwart the incursions," it said.
Earlier on Friday, Palestinian mobile phone service provider Jawwal said that services, including mobile and landline phones and internet, had been cut by heavy bombardment.
The cutoff meant that casualties from strikes and details of ground incursions could not immediately be known. Some satellite phones continued to function.
A statement from the Palestine Red Crescent Society said it had completely lost contact with its operations room in Gaza and all its teams operating on the ground.
World Health Organization chief Tedros Adhanom Ghebreyesus tweeted Saturday that his organization still has no communication with its staff and health facilities in Gaza.
In past violent flare-ups and wars, Israeli jets have hit parts of the massive tunnel network which Hamas has steadily developed and rebuilt for years, often referred to by Israeli soldiers as the "Gaza Metro."
John Spencer of the Modern War Institute at elite U.S. military academy West Point, described "a veritable city underneath the cities on Gaza's surface."
In a study released this month, Spencer said "tunnels will be the vital element of Hamas's guerrilla warfare strategy" against Israeli soldiers.
Israeli military officials say some shafts go as deep as 40 meters and can withstand bombs that weigh up to 455 kilograms.
As Israel pummels the narrow territory mostly from the air, Hamas bets on the relative safety of the tunnel network — parts of which have extended beyond Gaza's borders into Egypt and Israel — for its fighters and leaders, as well as to store weapons and basic supplies, according to experts.
Spencer called the tunnels a "wicked problem" for Israel's military, "for which no perfect solution exists".
Tunnels into Egypt have been used to smuggle arms, food and alcohol into Gaza, under a crippling Israeli-led blockade since Hamas took power in 2007.
Cairo had flooded some in a bid to destroy the network.
Some information for this article was sourced from Agence France Presse.