Mounting casualties in Gaza drew protests from European and Arab capitals and sent thousands of Palestinians into the streets across the West Bank, where Israeli troops killed a teenager during a demonstration.
The spike in violence during the past five days is a setback for U.S. Secretary of State Condoleezza Rice, who had hoped during her visit here this week to advance peace talks that President Bush helped launch in November with the aim of an accord on Palestinian statehood by the end of his term.
Instead, Rice faces the task of putting talks back on track.
Clashes between Israeli soldiers and Palestinian fighters have killed more than 100 Palestinians and two Israeli soldiers.
Nabil abu Rudeineh, a spokesman for Abbas, said Israel's "unjust war and open-ended massacre [in Gaza] is obstructing the peace process," making it untenable for Palestinian negotiators to meet with Israeli counterparts "until the aggression ends."
Israeli officials say the incursion by hundreds of troops with tanks and warplanes is intended to push Hamas and other militant groups away from the border, putting their rockets beyond range of Israeli communities that have been hit nearly every day for months.
Israeli Defense Minister Ehud Barak indicated yesterday that the assault was a rehearsal for a bigger operation that he said would try to "weaken Hamas rule and, in the right circumstances, even bring it down."
An Israeli attempt to forcibly depose Hamas rulers in Gaza would mark a significant change in Israeli policy, and Barak might have been speaking out of turn.
"There is no such policy," said Mark Regev, a spokesman for Israeli Prime Minister Ehud Olmert. "Obviously, in the long term, we would like to see the legitimate Palestinian government resume control of the Gaza Strip. Having said that, that is not one of our military objectives."
Hamas leaders dismissed Barak's threats as the latest in a string of counterproductive attempts to weaken the Islamist group's power.
"All the time they are talking about toppling Hamas, and every day Hamas is getting stronger," said Ahmed Yousef, a political adviser to Ismail Haniyeh, the Hamas leader who was the Palestinian Authority's prime minister until he was fired last summer. "With these bloody massacres they have committed they are making people stand with Hamas."
Abbas' secular-led government claims to speak for both Palestinian territories but controls only the West Bank. Gaza is ruled by his bitter enemies in Hamas, the Islamic group that rejects a peace settlement and is the target of Israel's incursion.
As much as he would like to see Hamas weakened, Abbas is careful to distance himself from Israel when it attacks Gaza. The peace talks have not achieved enough to boost his stature among Palestinians, and he fears losing credibility to militant groups that call for Israel's destruction.
In a show of solidarity with Gazans, Abbas declared a day of mourning yesterday and donated blood for Palestinians wounded in the fighting. He told reporters at his West Bank office that he had been contacting foreign leaders and seeking support.
The European Union and Turkey joined yesterday with Arab countries that have criticized Israel for using what they call disproportionate force in Gaza, where civilians account for more than a third of the scores of Palestinians killed since Wednesday.
The United Nations Security Council urged Israelis and Palestinians to "immediately cease all acts of violence." In Texas, National Security Council spokesman Gordon Johndroe told reporters, "The violence needs to stop and the talks need to resume."
Olmert rejected the criticism.
"No one has the moral right to preach to Israel for taking the elementary step of self-defense," he said, adding that attacking Hamas "strengthens the chance for peace."
"I'm sure that beyond certain statements, the Palestinian leadership, the one with whom we want to achieve peace, also understands that."
Despite the assault, the Israeli army said the militants fired at least 25 rockets into Israel yesterday, making direct hits on three houses and slightly wounding nine people.
Olmert came under criticism from both ends of Israel's political spectrum. Benjamin Netanyahu, leader of the right-wing Likud party, said the government must take stronger action against Hamas.
Speaking at the funeral of his nephew, one of two Israeli soldiers killed last week in the fighting, Yoel Mizrahi issued an anti-war appeal: "Gaza is not our country. Parents, wake up and stop the next tragedy. Do not send them to Gaza."
In the West Bank city of Hebron, Mahmoud Musalameh, a 14-year- old boy wearing a Hamas headband, was fatally wounded in the chest after Israeli troops opened fire on what a military spokesman called a dangerous Palestinian crowd that was protesting the incursion by throwing rocks and firebombs.
Richard Boudreaux writes for the Los Angeles Times. McClatchy-Tribune contributed to this article.