At least nine Palestinians have been killed in Israeli raids and strikes on several towns in the north of the occupied West Bank, a Red Crescent spokesperson said.
The number was revised after a spokesperson reported 10 people were killed in three locations.
The assault is one of the largest seen in the West Bank in months.
The spokesperson added that 15 others had been wounded.
Palestinian health officials said at least nine people had been killed. Other media reports put the death toll at 11.
IDF declines to say how long operation will last
Early on Wednesday, the Israeli army said it was carrying out an "operation to thwart terrorism in Jenin and Tulkarm" in the northern West Bank.
It later posted on X that it had killed nine people in the operation, claiming those killed were militants.
Military spokesman Nadav Shoshani also told reporters no Israeli soldiers were harmed during the operation, but declined to say how long it would last or how many troops were involved.
The operation targeted "a mixture of terror groups and terror cells" after the West Bank saw "a significant rise in terror activity in the past year", Mr Shoshani said.
In Jenin, Israeli forces "apprehended five wanted suspects and located and confiscated weapons" including rifle parts and ammunition, he said.
"We have met explosives already in the first hours, and we have met real-time fire exchanged with terrorists engaging in battle," he said.
"Additionally, the forces exposed and dismantled explosives that were planted under the roads in the area and were intended to be detonated in attacks against the security forces operating in the area."
The governor of Jenin said Israeli forces surrounded the city and blocked hospital access. (AP: Majdi Mohammed)
Foreign Minster Israel Katz added on X that the military was "operating in full force since last night" in a bid to "dismantle Iranian-Islamic terror infrastructure".
He accused Iran of seeking to "establish an eastern front against Israel".
"We must address this threat with the same determination used against terror infrastructures in Gaza, including temporary evacuation of residents and any necessary measures," he said.
"This is a war, and we must win it."
Hospital access points blocked during raids
By midday local time, the city of Jenin was relatively quiet but in the crowded refugee camp — a heavily built up township adjacent to the main urban area — occasional explosions could be heard.
The IDF said its decision to block hospital access points was aimed at preventing militants from seeking refuge. (AP: Majdi Mohammed)
Masoud Naaja, the father of two young men killed in a strike, said he was giving water to some men who asked for a drink when he was wounded.
"In seconds, very fast, we felt like something came down on us from the sky and there was an explosion," he said.
"When I put my hand on my chest, it was full of shrapnel and blood."
Governor Kamal Abu al-Rub of Jenin said on Palestinian radio that Israeli forces had surrounded the city, blocked exit and entry points and access to hospitals and ripped up infrastructure in the camp.
The Israeli military said the move was intended to prevent militants from seeking refuge.
The Palestinian Health Ministry said the bodies of seven people were brought to the hospital in Tubas, another West Bank city, and another two were brought to the hospital in Jenin.
The ministry identified two killed in Jenin as Qassam Jabarin, 25, and Asem Balout, 39.
Palestinian President Mahmud Abbas cut short a visit to Saudi Arabia and headed home to "follow up on the latest developments in light of the Israeli aggression on the northern West Bank", Palestinian official media said.
Israel attempting to annex West Bank, militants say
The governor of Jenin said Israeli forces ripped up infrastructure in the camp. (AFP: Jaafar Ashtiyeh)
The operation comes two days after Israel said it carried out an air strike on the West Bank that the Palestinian Authority reported killed five people.
The armed wings of Hamas, Islamic Jihad and Fatah said in separate statements their gunmen were detonating bombs against Israeli military vehicles as they fight Israeli forces in the three West Bank areas.
"With this aggression, which aims to transfer the weight of the conflict to the occupied West Bank, the occupier wants to impose a new state of affairs on the ground to annex the West Bank," read a Wednesday statement from Islamic Jihad.
It added its fighters were using machine guns in close range combat with Israeli troops and targeting military bulldozers with explosives.
Violence in the West Bank has surged alongside the war in Gaza, with more than 640 Palestinians in the territory killed by Israeli troops and settlers since Hamas's October 7 terrorist attack, according to an AFP tally based on Palestinian health ministry figures.
At least 19 Israelis have been killed in Palestinian attacks during the same period, according to Israeli officials.
During the October 2023 terrorist attack, Hamas militants killed 1,200 people and took more than 250 hostages, according to Israeli figures.
The militants are still holding some 110 hostages, around a third of whom are believed to be dead, after most of the rest were released during a November cease-fire.
Israel's Gaza campaign has since levelled swathes of the enclave, displaced nearly all its 2.3 million people multiple times, given rise to deadly hunger and disease and killed more than 40,400 people, Palestinian health officials say.
Internationally mediated talks to end the conflict continue, with Hamas and Israel trading blame for a lack of progress and the United States expressing optimism that a ceasefire can be reached.
There was no sign of a breakthrough after days of talks in Egypt, and the negotiations move to Qatar this week.
ABC/Wires