JERUSALEM — Hamas declared a formal end to its cease-fire with Israel on Thursday, ruling out an extension of a 6-month-old pact that had begun to fray weeks ago with tit-for-tat attacks across Israel's border with the Gaza Strip.
Fawzi Barhoum, a spokesman for the militant group that controls Gaza, said the truce would expire early today. He said it was not being renewed because "the enemy refused to comply" with promises to lift a crippling blockade of the Palestinian enclave.
The decision's immediate effect was unclear. Hamas stopped short of threatening an escalation of rocket and mortar attacks, and Israeli officials said they were reluctant to launch a major military offensive in the densely populated territory. The border remained quiet in the first hour after the truce lapsed.
But the collapse of the Egyptian-brokered accord dimmed hope of a long-term calm that could help Israel avoid friction with moderate Arab nations.
It raised the threat of fresh strikes on southern Israeli towns within rocket range of Gaza and a tighter squeeze on the coastal strip's 1.5 million Palestinians, already short of food, fuel, electricity and other essentials.
"The truce was always fragile, with numerous violations by each side from the beginning," said Robert A. Pastor, an American University professor who joined former President Carter in meeting with Hamas leaders in Syria last weekend and then traveled here without Carter to discuss the truce with Israeli officials.
Pastor said both sides still had an opportunity to revive the accord and expand it, "but this will become less likely if the pattern of rockets and retaliation continues."
Israel has sharply restricted movement across its border with Gaza since Hamas, whose charter calls for destroying the Jewish state, won the Palestinian Authority's parliamentary elections in early 2006 and launched near-daily rocket attacks from Gaza. The boycott was tightened after the Islamic group seized full control of the territory in June 2007, ousting the rival secular-led Fatah movement.
Hamas' truce with Israel was the first such accord to call for easing the blockade. It was conditioned on observance of a "mutual and simultaneous calm" by the Israeli army and Gaza's well-armed paramilitary units.
Although violence and casualties dropped sharply, neither side was satisfied. Hamas complained that Israel allowed far less than the promised restoration of cross-border deliveries to mid-2007 levels.
The United Nations and other observers supported this assertion and accused Israel of inflicting collective punishment on Gaza's civilian population.
According to U.N. data, the number of truckloads of food and humanitarian supplies reaching Gaza rose during the truce to a peak of 2,593 per month but remained far below the 9,400 monthly average before mid-2007.
Supplies of industrial diesel fuel also rose, to 676,000 gallons per week, but remained short of the 924,000 gallons needed each week to operate Gaza's only power plant at full capacity, according to Gisha, an Israeli group that advocates freer movement over the Gaza border.
Israeli officials countered that any shortages should be blamed on Hamas' unceasing belligerence. Although rocket and mortar attacks dropped to one a week during early autumn, they never stopped, and Israel said Hamas used the calm to replenish its arsenal by smuggling in weapons from Egypt.
The truce began to unravel Nov. 4 when Israeli forces entered Gaza for the first time since June to blow up a tunnel that, according to Israel, Hamas was digging as part of a plan to capture Israeli soldiers along the border. Six Hamas members were killed during the operation.
Since then, Hamas and smaller groups in Gaza have fired nearly 300 rockets and mortar shells into Israel, the Israeli military said. At least 10 more Palestinian militants have been killed in Israeli assaults.
For the first time since the truce took effect, Israeli fighter jets roared over Gaza on Thursday, targeting rocket launchers and arms depots.
Although the violence is less intense than it was before the truce, Israel has imposed a near-total closure of the border since early November, causing frequent electricity blackouts, food shortages and a depletion of cash reserves in Gaza banks. On Thursday, the U.N. Relief and Works Agency, which feeds half of Gaza's population, ran out of donated food and closed its warehouses.
Israeli officials said the unwritten truce agreement had no expiration date. Wary of a confrontation that could cause heavy casualties on both sides, they had hoped Hamas would agree to extend it.
But indirect talks brokered by Egypt failed last weekend to achieve a new understanding. Hamas said its demands for unrestricted movement of goods through the border were rejected.
Defense Minister Ehud Barak said Wednesday that Israel would react cautiously to the end of the truce.
"We are not afraid of launching a large-scale military operation in Gaza, but there is no need to rush into it," he said. "When the situation requires us to, we will act."
Special correspondent Rushdi abu Alouf in Gaza City contributed to this report.