Since the operation began early Thursday, 19 Palestinians and one Israeli soldier have been killed. Dozens of Palestinians were injured, doctors and witnesses said. The Israeli move drew stiff criticism from Palestinians and some Israelis.
The Israeli military said that the camps are headquarters for "terrorist members who have carried out attacks causing the murders of tens of innocent civilians."
In response, Palestinian leader Yasser Arafat called the operation, "a new massacre against the Palestinian people."
He urged the world on Friday to force Israel to halt the raids.
"I call upon the whole world to act quickly before a state of chaos engulfs the whole Middle East region," Arafat told reporters at his headquarters in the West Bank city of Ramallah.
Palestinian officials are accusing Israel of launching the operation to undermine a land-for-peace proposal floated by Saudi Crown Prince Abdullah that has won U.S. support and gained international momentum, CBS News Correspondent David Hawkins reports.
"We believe this is an intention to widen the war against the Palestinian people and these actions will bring retaliation," said Palestinian Information Minister Yasir Abd Rabbo. "And that's why (Israeli Prime Minister Ariel) Sharon is responsible for the vicious circle of violence we are living."
"There's no cycle of violence really here," countered Israeli government spokesman Raanan Gissin. "There's unmitigated violence by these unruly groups. If the Palestinian Authority would have done and fulfilled its obligations we would not have needed to go into the camp."
In other violence, a seven-year-old Palestinian boy was killed when Israeli soldiers opened fire on a village in the northern part of the Gaza Strip, residents and doctors said. The Israeli military was checking the report.
Israeli forces entered the refugee camp in Jenin around 4 a.m. Friday, witnesses said, after a full day of fighting in the Balata camp next to Nablus.
Shami Shami, a leader of Arafat's Fatah movement in the Jenin camp, said Israeli tanks surrounded the Jenin camp, and troops were confronting about 300 gunmen. Two Palestinian teen-agers, one a gunman and the other a Hamas backer, were killed and 30 injured in fighting Friday morning in the Jenin camp, residents and doctors said. Witnesses said Israeli soldiers were going from house to house making arrests.
"We saw a helicopter shooting at a house," Shami told The Associated Press by telephone. "The gunmen are shooting at the tanks and others are throwing homemade grenades," he said. "Others are planting bombs in the streets."
Many suicide bombers from the militant Hamas and Islamic Jihad have left for attacks in Israel from the camp next to Jenin, at the northern edge of the West Bank. The Balata camp is a stronghold of militias linked to Fatah.
Israeli military sources, speaking on condition of anonymity, said Balata was targeted because militia leaders in the camp were responsible for four recent attacks, including a suicide bombing by a woman Wednesday at an Israeli army checkpoint.
In the Balata camp, residents said the search operation was continuing. To stay out of alleyways controlled by snipers and militants with explosives, the soldiers went from house to house by knocking down walls between the flimsy shelters, residents said.
Ten houses were destroyed, residents said. Suleiman Abu-Asab, 52, said 12 soldiers broke down a door and entered two building where seven families, including 30 children, live.
"They spent all night there," he said. "When we returned in the morning after they left, we found the house badly damaged totally, much of the furniture burned and even their empty food boxes still inside."
Palestinian security officials said most of the gunmen had left the camp, and the Israeli soldiers were not making arrests. Instead, they said, it appeared that the soldiers were looking for weapons and explosives.
Residents said troops destroyed the house of Nasser Awais, commander of the Al Aqsa Brigades militia in the camp, the Fatah-linked group that has claimed responsibility for many recent attacks.
Israeli officials said their soldiers entered the two camps to go after the control stations of terrorists who have been plaguing Israelis throughout the conflict with bombing and gunfire attacks.
In Washington, U.S. State Department spokesman Richard Boucher said Israel has the right to defend itself, but he expressed concern over the incursions. "We have been in touch with the Israeli government to urge that utmost restraint be exercised in order to avoid harm to the civilian population," he said.
Israeli military commentator Alex Fishman heaped criticism on the entry into the camps, a measure the Israelis had avoided up to now because of concern over Israeli casualties.
"This is not a calculated risk, it's a dangerous gamble," Fishman wrote in the Yediot Ahronot daily. "One must be either stoned or a gambler to send the army in at this time and place," he wrote, adding that it meant that Israel had given up on an agreement to stop the violence.
The 27 refugee camps in the West Bank and the Gaza Strip house Palestinians displaced in Mideast wars dating to Israel's founding in 1948. Though officially designated as refugee camps, most are made up of concrete-block houses and apartment buildings that have gone up over a period of decades, considered temporary shelter for the refugees and three generations of descendants.