Israel says it has arrested 19 members of the Palestinian group Islamic Jihad in raids across the occupied West Bank, after carrying out airstrikes against targets in the Gaza Strip.
Fifteen people have been killed in the air strikes, including a child and PIJ leader Tayseer Jabari.
Israel said the strikes followed an “immediate threat” from the group.
Scores of rockets were fired from Gaza towards Israel overnight, most of them intercepted, Israel’s military said.
The latest violence is the most serious flare-up between Israel and Gaza in just over a year.
The 11-day conflict in May 2021 left more than 200 Palestinians and a dozen Israelis dead before a ceasefire was agreed.
The Israeli military is warning this latest operation – codenamed Breaking Dawn – could last for a week.
Sirens warning of incoming missiles continued to sound in Israeli towns on Saturday, amid more reports of air strikes in Gaza that Israel says are targeting PIJ militants.
Palestinian health officials confirmed a man was killed near Khan Younis in the south of the strip.
But so far Hamas, the biggest militant group in the area – which has similar ideology to Islamic Jihad and often coordinates its actions with it – does not seem to be firing from its large rocket arsenal.
As a result, there are no reports of Israeli air strikes targeting Hamas.
Hamas issued strong statements on Friday night saying that “resistance groups” were united. But because it governs Gaza it has its own practical considerations which may stop it from getting more involved.
Life in the Palestinian territory has already become much harder in the past week, after Israel closed its crossings with Gaza amid fears that Islamic Jihad would retaliate for the arrest of one of its leaders in the northern West Bank.
The calculations of Hamas could change, if for example the civilian death toll in Gaza rises rapidly.
If it does decide to join the fighting then it would quickly get much more intense.
If things stay like this, Egypt – which often acts as a go-between for Israel and Gaza – could have a better chance of brokering some kind of truce.
Cairo officials were preparing on Saturday to host a potential delegation of PIJ representatives as part of that process, Egyptian media said.
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Almost 200 rockets were fired from Gaza towards Israel overnight, the Israeli military said. Most were intercepted by the Iron Dome defence, with no Israeli casualties.
Some 30 Islamic Jihad targets have been struck, among them two weapon storage facilities and six rocket manufacturing sites, the IDF said. More than 120 people have been injured, the Palestinian health ministry said.
Referring to the initial strike on Friday, Israeli Prime Minister Yair Lapid said Israel carried out “a precise counter-terror operation against an immediate threat”.
Meanwhile, Interior Minister Ayelet Shaked said: “We don’t know how this will play out… but this could take time… this could be a lengthy round [of conflict] and a hard one.”
The IDF said its attacks targeted sites linked to PIJ, including the high-rise Palestine Tower in Gaza City which was hit on Friday in a loud explosion which left smoke pouring from the building.
The IDF said Tayseer Jabari was a “senior commander” in PIJ, and accused him of having committed “multiple terrorist attacks” against Israeli civilians.
Five-year-old Alaa Kaddum was among those killed in the strikes, local officials also said.
In remarks delivered while on a trip to the Iranian capital Tehran, PIJ secretary general Ziyad al-Nakhala said: “We will respond forcefully to this aggression, and there will be a fight in which our people will win.
“There are no red lines for this battle… and Tel Aviv will be under the rockets of the resistance.”
The latest conflict closely follows Israel’s arrest of Bassem Saadi, reported to be the head of PIJ in the West Bank, on Monday night.
He was held in the Jenin area as part of an ongoing series of arrest operations after a wave of attacks by Israeli Arabs and Palestinians that left 17 Israelis and two Ukrainians dead. Two of the attackers came from the Jenin district.