The Israeli military said all troops were withdrawn from Gaza in the hours before the 6 a.m. cease-fire, announced late Saturday, went into effect. Dozens of tanks and armored vehicles were parked just over the border in a military staging ground in southern Israel early Sunday, and the streets of northern Gaza were empty.

But occasional rocket and mortar fire from Gaza continued to strike Israel within the truce's first hour.

"Let's hope that's just the problems of the beginning," Israeli government spokeswoman Miri Eisin said. "But if Israel is attacked, we will respond. If there are Palestinian factions that are not part of the cease-fire, it's hard to see how the cease-fire will hold."

All militant factions denied involvement in the attacks.

A spokesman for the Palestinian government, Ghazi Hamad, said all armed groups have committed to the agreement, and termed any violations rogue acts.

"There is 100 percent effort to make this work, but there is no guarantee of 100 percent results," Hamad said.

The truce agreement, if it holds, will be a coup for Palestinian President Mahmoud Abbas as he tries to form a more moderate government than the one currently led by Islamic Hamas radicals, who refuse to recognize Israel. Abbas, a moderate from the Fatah Party elected separately last year, hopes a new government lineup will persuade the West to end crushing economic sanctions imposed after Hamas took power in March.

The cease-fire was meant to halt both rocket fire and other militant attacks on Israel from Gaza, and a military offensive Israel launched in the coastal strip in June, less than a year after ending its 38-year occupation.

The two sides announced the accord after Abbas telephoned Israeli Prime Minister Ehud Olmert late Saturday to tell him he wrested an agreement from Palestinian factions to stop rocket fire and all other violence from Gaza. Olmert reciprocated by pledging to stop all Israeli military operations in Gaza and withdraw all troops there.

Abbas spokesman Nabil Abu Rdeneh said a truce reached in Egypt in February 2005 would be revived.

"There is a signed agreement between the president and Prime Minister (Ismail) Haniyeh and all the Palestinian factions to resort to the agreement of the factions in Cairo in 2005, including ceasing all the military activity from Gaza, starting from Sunday morning," Abu Rdeneh said from Gaza City. "The Israeli prime minister has agreed, and it is going to start tomorrow morning."

Abbas told Olmert that the factions had agreed to stop all violence from Gaza, including rocket fire and suicide bombings, starting at 6 a.m. Sunday, Eisin said.

He "asked that, in response, Israel stop all military operations in the Gaza Strip and withdraw all her forces," and Olmert responded favorably, she said.

Israeli forces originally entered Gaza to try to recover a soldier Hamas-linked militants captured in a June 25 cross-border raid. But they soon widened their objectives to target militants who had intensified their rocket attacks on southern Israel after the September 2005 Gaza pullout.

The violence claimed the lives of more than 300 Palestinians and five Israelis. Most of the dead Palestinians were militants, but dozens of civilians died, too, including 19 members of an extended family killed earlier this month.

Despite international criticism over Palestinian civilian deaths, Olmert pledged to continue the offensive until Palestinian rocket attacks from Gaza significantly decreased. Instead, as the fighting swelled, rocket fire in November more than doubled from October, killing two Israeli civilians in a single week.

The militants' capture of Cpl. Gilad Shalit, who remains in captivity, and the subsequent Israeli incursion into Gaza cut short efforts by Olmert and Abbas to restart peace talks that broke down six years ago. A truce could help to create the momentum to get talks moving.

"We welcome the announcement and see this as a positive step forward," White House spokesman Alex Conant said Saturday evening in Washington. "We hope it leads to less violence for the Israeli and Palestinian people."

Although Israel has no ties with the Hamas government, which rejects the Jewish state's right to exist, it considers the separately elected Abbas an acceptable negotiating partner. He and Olmert agreed months ago to meet, but Abbas has balked at setting a date without receiving assurances the meeting would yield real dividends for him, like a release of Palestinian prisoners Israel holds.

Olmert has said no prisoners would be released to Hamas before Shalit is freed.

A cease-fire in Gaza is part of a broad package Abbas is trying to put together in the hope of restoring hundreds of millions of dollars in funding Western powers cut off to pressure Hamas to recognize Israel and renounce violence.

The centerpiece of that package would be the formation of a new government less inimical to Israel. Another major element is a prisoner swap.

Hamas' supreme leader, Khaled Mashaal, arrived in Cairo this week to discuss both issues with Egytian mediators, but there was no word of a breakthrough.

On Saturday, Mashaal said his group was willing to give peace negotiations with Israel six months to reach an agreement for a Palestinian state in Gaza and the West Bank, but threatened a new armed uprising if the talks failed.

The double-edged comments were his strongest confirmation that the Islamic militant group would allow Abbas to try to negotiate with Israel. But it was also the first time he has set a deadline with an explicit threat of a new uprising.

Israel had no immediate comment on Mashaal's proposal.