Israel brushed off past criticism yesterday of its cease-fire agreement with Hamas and reportedly moved to tackle other diplomatic fronts, including negotiations with Lebanon over a strategic plateau and a possible prisoner exchange with Hezbollah.

Prime Minister Olmert denied that the Gaza pact gave legitimacy to Hamas, the Islamist group that controls the territory, and said Israel would hold the organization responsible for any violation of the cease-fire. Hours before the agreement was scheduled to take effect, Mr. Olmert called it “fragile” and said it could be “short-lived.”

On a visit to Israel last week, Secretary of State Rice secured a promise from Mr. Olmert to negotiate with Lebanon over Shebaa Farms, a sliver of land on hills abutting Lebanon, Syria, and Israel, Haaretz reported. The dramatic policy shift from Washington, which had pressured U.N. officials to drop the Shebaa issue, was aimed at strengthening the Lebanese prime minister, Fouad Siniora, whose political fortunes have ebbed since Hezbollah’s recent display of military force in Beirut.

Mr. Siniora, however, said last night that his country will not negotiate bilaterally with Israel.

News reports from the region also predicted a prisoner swap between Israel and Hezbollah as early as next week. Such an exchange could include the two soldiers whose kidnapping in July 2006 sparked a war between Israel and Hezbollah, Eldad Regev and Ehud Goldwasser.

A month earlier, Hamas kidnapped Gilad Shalit, who is said to be alive and held in Gaza. Mr. Olmert has promised the Israeli corporal’s parents, Aviva and Noam Shalit, that Israel’s economic blockade against the Palestinian Arab territory will not be lifted without “an arrangement assuring the return of Gilad,” the Shalits’ lawyers wrote in a letter to the prime minister.

The Shalits see the Israel-Hamas agreement as breaking that promise. The truce is not only “morally outrageous and extremely unreasonable, but also illegal,” the lawyers wrote.

“This is one of the worst agreements in our history,” a Likud member of the Knesset’s defense and foreign relations committee, Yuval Steinitz, told The New York Sun. “Israel agrees, de facto, to violate its own policy of isolating Hamas.”

Hamas now will be able to smuggle missiles into Gaza that soon could hit Tel Aviv, just as Hezbollah bypassed agreements meant to ensure that it would not rearm, Mr. Steinitz said. He also predicted that Hamas would be able to install intelligence equipment supplied by Iran, allowing Tehran to maintain the electronic surveillance of military installations across southern Israel, as it does now in the north via equipment installed by Hezbollah.

“We have no illusion but that this truce is fragile and could be short-lived,” Mr. Olmert told supporters at a conference north of Tel Aviv. The agreement reportedly includes an immediate cessation of shooting this morning, to be followed by a gradual easing of the blockade Israel has maintained on Gaza since Hamas took over the territory last year. Meanwhile, Israel and Hamas will continue to negotiate, with Egypt as a mediator, vis-à-vis Corporal Shalit and arrangements to prevent weapons smuggling into Gaza.

The Israel Defense Force has received orders to be ready, at any moment, to conduct “any operation that might be needed,” Mr. Olmert said, referring to preparations for a large ground incursion into Gaza. However, “if the cease-fire collapses, it will not be because of us,” Mr. Olmert’s spokesman, Mark Regev, told the Sun.

On Tuesday, critics of the agreement, including a top adviser on the Middle East for Senator Obama, Daniel Kurtzer, said a pact with Hamas would sideline the president of the Palestinian Authority and Israel’s and Washington’s favored diplomatic interlocutor, Mahmoud Abbas, also known as Abu Mazen.

“We don’t have a lot of alternatives,” a spokesman for the Israeli Foreign Ministry, Arye Mekel, said. “If we attacked Gaza militarily, Abu Mazen would have immediately withdrawn from the talks. He prefers the calm.”