Israeli troops killed nine Palestinians, including an elderly woman, and wounded 12 others during clashes with gunmen on Thursday in a flashpoint town in the occupied West Bank, witnesses and medics said.
The Israeli military confirmed that an operation was under way in Jenin but did not immediately provide further details.
The Palestinian armed groups Hamas and Islamic Jihad said their gunmen were battling troops who had swept into Jenin refugee camp, a militant bastion where such deep raids are rare.
At the entrances to the camp's cramped alleyways, local youths threw rocks at army vehicles. Gunfire echoed as well as occasional explosions as militants set off improvised bombs.
Jenin is an area of the northern West Bank where Israel has intensified raids over the past year after local men carried out street attacks in its cities. The violence has further clouded stalled US-sponsored talks on Palestinians statehood.
The Palestinian health ministry described the nine fatalities in Jenin as an elderly woman and eight men. There were no further details immediately available on their identities.
An Islamic Jihad official told Reuters that the group had contacted mediators of past truces with Israel to warn that the Jenin violence could create escalation elsewhere.