As operatives of Hamas boasted victory, claiming an agreement with Israel to temporarily halt violence in Gaza is imminent, American and Israeli critics, including a top Middle East adviser to Senator Obama, Daniel Kurtzer, expressed skepticism that the lull in fighting could hold for long.

Reports from Cairo and Gaza said the agreement, completed after weeks of indirect Egyptian mediation efforts between Israel and Hamas, will be announced officially as early as tomorrow. Israeli officials, who have teetered for days between saying an agreement to reach calm is imminent and, conversely, that a major military operation in Gaza may be launched soon, declined to confirm that they signed on to a cease-fire pact.

A “mutual and simultaneous calm” will take effect tomorrow, the Egyptian news agency MENA reported yesterday, citing an unidentified government official. The tahadia — Arabic for a temporary period of calm — would be the first phase of a more comprehensive agreement, the official said.

A top Hamas operative in Gaza, Mahmoud a-Zahar, said the tahadia agreement is for a six-month period in which both sides would cease fire. A swap of Arab prisoners for a kidnapped Israeli soldier, Gilad Shalit, who is believed to be held by Hamas, would not be included in the pact and would involve a separate set of negotiations, Mr. a-Zahar said.

A leading dovish member of Israeli Prime Minister Olmert’s cabinet, Vice Premier Haim Ramon, publicly expressed opposition to signing the agreement with Hamas. “This is yet one more victory for radical Islam. It was victorious in Lebanon, and now in Gaza,” he said, according to Israeli press reports.

Nevertheless, signaling that discussions with Egypt have reached a critical point, the Israeli defense ministry’s top Gaza negotiator, Amos Gilad, flew to Cairo yesterday for an unscheduled meeting with President Mubarak’s intelligence chief, Omar Suleiman, who has presided over the indirect negotiations.

The Israeli army chief of staff, Lieutenant General Gabi Ashkenazi, told the Knesset’s Foreign Affairs and Defense Committee earlier that a cease-fire with Hamas would be “short and fragile.” In recent weeks, General Ashkenazi reportedly became increasingly supportive of a major military incursion into Gaza, which would be designed to diminish the terrorists’ ability to shoot rockets across the border into Israeli towns.

“I am skeptical,” Mr. Kurtzer, a foreign policy adviser for the Illinois senator who is the presumptive Democratic nominee, told The New York Sun as he briefed the American Jewish group Israel Policy Forum yesterday. Hamas cannot be trusted to keep a cease-fire, Mr. Kurtzer said, and even if it does, the agreement would not necessarily commit other splinter groups to end attacks on Israel.

His caution was echoed by members of the Bush administration. “Saying you have a loaded gun to my head but you are not going to fire it today is far different than taking the gun down, locking it up, and saying you’re not going to use it again,” a State Department spokesman, Tom Casey, said. “Even if this is in fact a true report, it hardly takes Hamas out of the terrorism business,” he said, adding, however, that Washington supports Egyptian efforts to achieve “calm.”

Mr. Kurtzer, a former American ambassador to Egypt and Israel, sharply criticized President Bush’s record on Israeli-Arab peacemaking. He expressed concern that an agreement with Hamas would strengthen a group defined by Israel and America as a terror organization, while weakening the president of the Palestinian Authority, Mahmoud Abbas, who has been involved in peace talks with Israel under an American initiative known as the Annapolis process. “You don’t want to empower Hamas,” he said, noting that Mr. Abbas has urged American and Israeli officials not to do “anything to empower Hamas.”

In Ramallah, however, Mr. Abbas issued a statement in support of the reported agreement, and officials of his government were careful not to disparage it publicly even as Hamas, which has fought Mr. Abbas and resisted the peace talks he conducts with Israel, was sure to gain popularity in Gaza and beyond. “Hamas is in control of Gaza, and we need to think about the 1.5 million people of Gaza,” the Palestinian Arab U.N. observer, Riyad Mansour, told the Sun, saying the agreement should include a lifting of Israel’s blockade there.

“Hamas is exhausted and is requesting a truce due to the blockade Israel imposed on the Gaza Strip,” Israel’s president, Shimon Peres, told Israel Radio.

But a victorious Mr. a-Zahar gathered a press conference, telling Gaza reporters that the agreement signified a major vindication of his Islamic group’s policy of armed “resistance,” as well as a failure for Israel’s policy of isolating Gaza. “We have agreed to a mutual cease-fire between the two sides,” Mr. a-Zahar said, according to wire reports. “We don’t trust them, but we will see what happens,” he said of the Israelis.