JERUSALEM - Hamas declared a formal end to its cease-fire with Israel yesterday, ruling out an extension of a six-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 extremist group that governs Gaza, said the truce was expiring at 6 a.m. 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.

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 essential goods.

"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 Jimmy Carter in meeting with Hamas leaders in Syria last weekend and then traveled 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 ever 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 Hamas 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 during the cease-fire, neither side was satisfied. Hamas complained that Israel allowed far less than the promised restoration of cross-border deliveries to mid-2007 levels.

According to U.N. data, truckloads of food and humanitarian relief supplies reaching Gaza rose during the truce to a peak of 2,593 a month, far below the 9,400 monthly average that arrived before mid-2007.

Supplies of industrial diesel fuel also rose, to 676,000 gallons per week, but remained short of the 924,000 needed 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 Hamas' unceasing belligerence should be blamed for any shortages.