U.S. Assistant Secretary of State David Welch had been scheduled to return to Washington after talks in Beirut on Wednesday but changed his plans, Western diplomats said.

A senior Israeli official, who spoke on condition of anonymity, said the government postponed the assault for at least 24 hours "because we thought there was some hope" for a diplomatic deal. But the official said the postponement would be reviewed starting from this evening.

"If there is no diplomatic solution, there will be a military solution," Defence Minister Amir Peretz said during a visit to army headquarters in north Israel. Seconds later, sirens sounded and the minister and top generals entered a bomb shelter.

Western diplomats said they doubted Israel would delay the offensive beyond this weekend if an agreement is not reached on a U.N. Security Council resolution.

Prime Minister Ehud Olmert's security cabinet on Wednesday approved a plan to send troops up to 20 km (13 miles) into Lebanon, an operation officials said could take 30 days or more.

Under pressure from its key U.S. ally, Israel agreed to allow more time for U.S.-led diplomatic efforts before launching the new offensive, aimed at pushing Hizbollah out of southern Lebanon and stopping rocket fire into northern Israel.

"Fifteen casualties in one day proves what price we could pay if we do not try to make the most of the political move," Deputy Prime Minister Shimon Peres told Israeli Web site YNET, referring to Israeli soldiers killed on Wednesday.

Welch planned to meet senior Israeli officials, including Foreign Minister Tzipi Livni, who was considering a trip to New York if it looked like a deal could be reached.

Diplomats said U.S. and Israeli officials were discussing what they described as a "consecutive ceasefire" under which Israeli military operations would be scaled back in stages when a stabilisation force and the Lebanese army begin deploying.

Lebanon wants an immediate ceasefire and a quick pullout of Israeli troops from the south, where it says 15,000 Lebanese soldiers backed by U.N. peacekeepers can move in.

Israel, backed by Washington, says it will withdraw only if a foreign force is deployed to keep Hizbollah out of south Lebanon.

One Western diplomat described Israel's announcement of a new offensive without carrying it out as a "clever ploy" to step up pressure on the Security Council.

But analyst Mouin Rabbani of the International Crisis Group doubted that the threat would lead to a diplomatic breakthrough. "The more likely scenario is further escalation," he said.

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