American officials say the decision to rush the delivery of the weapons followed an Israeli request made around the time it began bombing Lebanon by air and sea on July 12.
"It's part of an arms sale that was made in 2005," said one, adding that the shipment "has either been made or is in the process of being delivered".
He refused to specify whether it was being delivered by land or by sea.
Last year's arms deal allows Israel - which agreed to purchase about one $US one billion dollars ($A1.4 billion) in US military equipment last year, according to a Congressional report - to tap into the package depending on need.
It was not immediately clear if Israel requested the new shipments following the start of the Lebanon campaign or while it was still in the peak of an air and ground offensive in the Palestinian Gaza Strip.
The US official said the shipment was negotiated by Israeli officials with the State Department and other agencies that have also refused to make public comment, highlighting the sensitivity of the decision.
"We do not, under a law, disclose sensitive information about foreign military sales," a Pentagon spokesman said.
"Disclosure of when weapons were shipped or if they were shipped ... gets into the order of battle of the country to which they were being sold," he added.
A State Department official said: "We are not making any comment on this at all".
The White House referred questions to the Pentagon and State Department.
The New York Times reports that the decision to expedite weapons was made with relatively little debate within the administration of President Bush.
But it noted the disclosure threatened to anger Arab governments and others because of the appearance that the United States is actively aiding the Israeli bombing campaign in a way that could be compared to Iran's efforts to arm and resupply Hezbollah.
Israel's request for rush delivery of the satellite- and laser-guided bombs was described as unusual by some military officers and as an indication that Israel still had a long list of targets in Lebanon to strike, The New York Times noted.
The US official refused to describe in detail the size and contents of the shipment, but confirmed that they included precision guided bombs.
The package also provides for satellite-guided munitions, The New York Times said.