The Israeli army has launched a massive military operation on the Jenin camp in the north of the occupied West Bank.
It started with multiple drone strikes. Videos published on social media showed smoke rising from a residential block.
The Palestinian Health Ministry said at least three Palestinians were killed and more than 20 others wounded in the ongoing raid.
The Israel Defense Forces (IDF) said they were striking "terrorist infrastructure in the Jenin area".
"We will not stand by idle while terrorists continue to harm civilians using Jenin Camp as a hideout," they said, adding that the camp was a "terrorist stronghold".
The Jenin Battalion, a military umbrella that includes armed men from Palestinian groups Fatah, Hamas and Islamic Jihad, said: "We will fight the occupation forces until the last breath and bullet, and we work together and unified from all factions and military formations."
Dozens of armed, masked Palestinians were deployed in the alleys of the camp.
Ahmed Zaki, a resident of the camp, told the BBC that "columns of Israeli army vehicles penetrated the outskirts of the camp from several streets".
Palestinian ambulance driver Khaled Alahmad said: "What is going on in the refugee camp is real war."
"There were strikes from the sky targeting the camp, every time we drive in around five to seven ambulances and we come back full with injured people," he told Reuters.
Last month, the Israeli military killed at least six Palestinians during a raid into the Jenin camp, the first time in years that it has used attack helicopters in the West Bank.
Seven Israeli soldiers and border police officers were wounded in that attack.