The offensive has also prompted concern that the deepening conflict could consume Beirut's fragile democracy.
Israel's actions have handed President George W Bush a new foreign policy crisis, as he grapples with raging violence in Iraq, and nuclear and missile belligerence from Iran and North Korea.
But analysts say that while Washington has been adamant in support of Israeli offensives in Gaza and Lebanon, wider US policy goals could be bruised if the situation gets worse.
"Israel has a right to defend herself," Mr Bush said earlier, after meeting German Chancellor Angela Merkel.
But he said that "whatever Israel does should not weaken" the government of Lebanese Prime Minister Fuad Siniora.
"Democracy in Lebanon is an important part of laying a foundation for peace in that region," Mr Bush said.
The US President also turned on Syria, which, along with Iran, the US suspects of orchestrating the kidnappings of the Israeli soldiers.
"Syria needs to be held to account," Mr Bush said. "President Assad needs to show some leadership towards peace."
Endorsement and warning
Analysts say they were both an endorsement of Israeli action and a firm warning to Syria that the United States would not stand idly by.
"I think that it is both -- it is not just tacit approval, that is certainly a clear approval. He was issuing a veiled threat to Syria," said
Edward Walker, a former ambassador to Israel and Egypt, now president of the Middle East Institute.
"Everybody in that part of the world has now become accustomed to the idea that the United States is now a paper tiger - it is so bound down in Iraq - and so they can act with impunity," he said.
James Philips, an analyst with the Heritage Foundation, which has close ties to the Bush administration, said Mr Bush sent a blunt message to Damascus.
"It is a sign to Syria by President Bush that he is not going to restrain
Israel," said Mr Philips. He said that a warning was implicit that Syrian targets could also come under the attack in the Israeli offensive.
Some Washington analysts shared international concern over the Israeli decision to extend its action to air raids on Beirut's airport, and a blockade of the city's port.
"The United States can't sit around and let civilian airports be targeted," said Mr Walker. But, he said, "We are nowhere near the point where the administration is going to try to rein in Olmert."
"The only way to rein in Olmert is to have a very active US policy that targets Iran and Syria and tries to build an international coalition behind a resolution of this problem."
Mr Philips said that would be a focus of the president this weekend at the G8 summit in Russia.
However, some observers worried that Washington's refusal to criticise the Israeli tactics could harm Washington's long-term goals in the Middle East.
They include a two-state solution to the Israeli-Palestinian conflict, building trust among Arab populations, and extricating US forces from Iraq.