Palestinian media reported that on Tuesday there were several air strikes as a large number of troops entered the city and its refugee camp, backed by drones, helicopters and armoured bulldozers.
Israel's prime minister said it launched an "extensive and significant" operation to "defeat terrorism" in Jenin, long seen as a stronghold of Palestinian armed groups.
It comes three days after the start of a ceasefire in Gaza and highlights the threat of more violence in the West Bank, where suspected Israeli settlers also went on the rampage on Monday night.
Jenin's governor, Kamal Abu al-Rub, told AFP news agency on Tuesday morning that "what is happening is an invasion of the camp", adding: "It came quickly, Apache [helicopters] in the sky and Israeli military vehicles everywhere."
Palestinian security personnel reportedly withdrew from some of their positions around Jenin refugee camp before the Israeli forces moved in.
The official Palestinian news agency, Wafa, later cited local sources as saying that Israeli forces were "completely besieging" Jenin camp, and that armoured bulldozers had dug up several streets.
The director of Jenin's Government hospital, Dr Wissam Bakr, also told the agency that three doctors and two nurses were among those wounded by Israeli gunfire.
A medical worker at another hospital, al-Amal, told the British charity Medical Aid for Palestinians: "I was called in [on Tuesday] due to the surge of injuries, and the moment I stepped out of the car making my way to the emergency entrance a sniper shot me below the knee."
"Another doctor was shot in the same spot," they added. "The Israeli army is shooting non-stop."
On Wednesday, Jenin's governor said Israeli forces had "bulldozed all the roads leading to Jenin camp, and leading to Jenin Government hospital".
Dr Bakr said earth mounds were blocking the entrances to the hospital, making it difficult to enter and leave, and preventing ambulance crews from reaching it.
The Israeli military said it was checking the reports.
The Palestinian health ministry reported on Tuesday evening that nine men and a 16-year-old boy, whom it named as Mutaz Abu Tbeikh, had been killed by Israeli forces in Jenin.
Another man was shot and killed by Israeli troops in the village of Tianik, about 8km (5 miles) to the north-west, it added.
Sources told the BBC that most of those killed were civilians.
However, the Israeli military and Shin Bet security service said on Wednesday that they had "hit over 10 terrorists" during the operation.
"Additionally, aerial strikes on terror infrastructure sites were conducted and numerous explosives planted on the routes by the terrorists were dismantled," they added.
Israeli Prime Minister Benjamin Netanyahu said in a statement that the Jenin operation - dubbed "Iron Wall" - was an "additional step in achieving the objective we have set: bolstering security" in the West Bank.
"We are acting methodically and with determination against the Iranian axis wherever it reaches: in Gaza, Lebanon, Syria, Yemen, and [the West Bank] - and we are still active."
Israel accuses Iran of smuggling weapons and funds to Hamas, Palestinian Islamic Jihad and other armed groups in the West Bank to foment unrest.
The prime minister of the West Bank-based Palestinian Authority, Mohammed Mustafa, condemned the raid, saying it was the latest in a series of "aggressive Israeli measures" against Palestinians in the West Bank, according to Wafa.
Hamas and Palestinian Islamic Jihad both called on Palestinians in the West Bank to escalate attacks against Israeli forces in response to the Jenin operation.
There have been a number of previous Israeli military operations in Jenin.
And recently, the PA's security forces carried out a controversial, weeks-long operation against armed groups there, including Hamas and PIJ, trying to reassert their control.
There has been a spike in violence in the West Bank since Hamas's deadly attack on Israel on 7 October 2023 and the ensuing war in Gaza.
Hundreds of Palestinians have been killed as Israeli forces have intensified their raids, saying they are trying to stem deadly Palestinian attacks on Israelis in the West Bank and Israel.
In another development in the West Bank on Monday night, dozens of masked Israeli extremists attacked Palestinians in two villages east of Qalqilya, Jinsafut and al-Funduq, setting fire to Palestinian homes and cars and smashing property.
At least 21 Palestinians were injured, the Palestinian Red Crescent said. Two Israelis were also shot, apparently when an Israeli police officer opened fire while responding to the violence.
"If you were there last night, you would not hear anything more than screaming of women and children," said Mohammed, whose family home in al-Funduq was metres away from a garden centre that was torched.
"In the end, we have don't have anything to protect ourselves. But they have everything to attack us."
The Israeli military said it was investigating the incidents, during which it said Israeli civilians "instigated riots, set property on fire, and caused damage". It also said they hurled stones and attacked Israeli security forces.
It happened just as new US President Donald Trump announced he was lifting sanctions on Israeli settlers accused of carrying out attacks in the West Bank.
The reversal of the Biden administration's sanctions targeting radical Israelis, could indicate the direction for the new White House that is expected to be more tolerant of Jewish settlement expansion.
The far-right, pro-settler Israeli Finance Minister, Bezalel Smotrich, welcomed the US move. In a post on X, he praised Trump's "unwavering and uncompromising support for the state of Israel".
Meanwhile, Palestinian officials condemned the change in policy. "Lifting sanctions on extremist settlers encourages them to commit more crimes against our people", the Palestinian foreign ministry said in a statement.
The mayor of al-Funduq, Luay Tayyam, told the BBC: "It is like a green light for the settlers, saying: 'Just go ahead, do whatever you want. You will not be persecuted.'"
"So they are happy with this news. And I think this was a big push for them last night. They feel encouraged by it."
There are also rising tensions over the large-scale release of Palestinian prisoners in the West Bank this week, as part of the Gaza ceasefire and hostage release deal between Israel and Hamas.
The attack in al-Funduq was in an area where three Israelis were killed in a shooting earlier this month. It was the latest in a long series of settler attacks that have accelerated markedly since the start of the Gaza war.
According to the Israeli anti-settlement group, Peace Now, in 2024 settlers also established 59 new outposts, without authorisation from the Israeli government. That was more than double the number from the previous year – which was also a record year for settlement outpost establishment.
Israel has built about 160 settlements housing some 700,000 Jews since it occupied the West Bank in the 1967 Middle East war. The settlements are considered illegal under international law, although Israel disputes this - as did the previous Trump administration.
