The airstrike, which was carried out early Friday and was confirmed by U.S. officials, comes as Washington considers how to respond to indications that the Syrian regime may have used chemical weapons in its civil war. President Barack Obama has described the use of such weapons as a "red line," and the administration is weighing its options including possible military action.
Israel has said it wants to stay out of the brutal Syria war, but could inadvertently be drawn in as it tries to bolster its deterrence and prevent sophisticated weapons from flowing from Syria to Hezbollah or other extremist groups.
Israel and Hezbollah fought a monthlong war in mid-2006 that ended in a stalemate.
Israel believes Hezbollah has restocked its arsenal with tens of thousands of rockets and missiles, and Prime Minister Benjamin Netanyahu has repeatedly stated the Jewish state would be prepared to take military action to prevent the Islamic militant group from obtaining new weapons that could upset the balance of power.
It is especially concerned that Hezbollah will take advantage of the chaos in neighboring Syria and try to smuggle advanced weapons into Lebanon. These include anti-aircraft missiles, which could hamper Israel's ability to operate in Lebanese skies, and advanced Yakhont missiles that are used to attack naval ships from the coast.
While Israeli officials on Saturday portrayed the latest airstrike as the continuation of Israel's deterrence policy, more Israeli attacks could quickly lead to an escalation, leaving open the possibility of retaliation by Hezbollah or even the Assad regime and Syria ally Iran.
In January, Israeli aircraft struck a shipment of what was believed to be Russian-made SA-17 anti-aircraft missiles bound for Hezbollah, according to U.S. officials. Israeli officials have strongly hinted they carried out the airstrike, though there hasn't been formal confirmation.
Neither Hezbollah nor Syria responded to that strike.
In a warning to Israel earlier this week, Hezbollah chief Hassan Nasrallah said his militia "is ready and has its hand on the trigger" in the event of an Israeli attack on any targets in Lebanon.
Details about Friday's strike remained sketchy.
The U.S. officials said the airstrike apparently hit a warehouse, but gave no other details.
Israeli officials did not say where in Syria the Israeli aircraft struck or whether they fired from Lebanese, Syrian or Israeli airspace.
Israel possesses bombs that can travel a long distance before striking their target. The use of such weapons could allow Israel to carry out the attack without entering Syrian skies, which would risk coming under fire from the regime's advanced, Russian-made anti-aircraft defenses.
The Israeli and U.S. officials spoke anonymously because they had not been given permission to speak publicly about the matter.
Obama said Saturday it was up to Israel to confirm or deny any strikes, but that the U.S. coordinates very closely with Israel.
"The Israelis, justifiably, have to guard against the transfer of advanced weaponry to terrorist organizations like Hezbollah," Obama told the Spanish-language TV station Telemundo.
The Syrian government said it had no information on an Israeli attack, while Hezbollah and the Israeli military spokesman's office declined comment.
Amos Gilad, an Israeli defense official, would not confirm or deny the airstrike, but played down cross-border tensions.
Hezbollah has not obtained any of Syria's large chemical weapons arsenal and is not interested in such weapons, Gilad said. Instead, the militia is "enthusiastic about other weapons systems and rockets that reach here (Israel)," he said Saturday in a speech in southern Israel.
Assad "is not provoking Israel and the incidents along the border (between Syria and the Israeli-controlled Golan) are coincidental," Gilad said.
After Hezbollah's military infrastructure was badly hit during the 2006 war, the group was rearmed by Iran and Syria with Tehran sending the weapons and Damascus providing the overland supply route to Lebanon.
"This is a very sophisticated network of Iranian arms, Syrian collection, storage, distribution and transportation to Hezbollah," said Salman Shaikh, director of The Brookings Doha Center and in 2007 involved in U.N. weapons monitoring in Lebanon.
Shaikh said Israel had detailed knowledge of weapons shipments to Hezbollah at the time and most likely has good intelligence now. "The Israelis are watching like hawks to see what happens to these weapons," he said.
With Israel apparently enforcing its red lines, much now depends on the response from Hezbollah and Syria, analysts said.
Israeli officials have long feared that Assad may try to draw Israel into the civil war in hopes of diverting attention and perhaps rallying Arab support behind him.
But retaliation for Israeli airstrikes would come at a high price, said Moshe Maoz, an Israeli expert on Syria.
"Bashar has his own problems and he knows that conflict with Israel would cause the collapse of his regime," Maoz said. "He could have done that long ago, but he knows he will fall if Israel gets involved."
Hezbollah, which is fighting alongside Assad's troops, appears to have linked its fate to the survival of the Syrian regime. Nasrallah, the Hezbollah chief, said this week that Syria's allies "will not allow Syria to fall into the hands of America or Israel."
On the other hand, Hezbollah could endanger its position as Lebanon's main political and military force if it confronts Israel, and it's not clear if the militia is willing to take that risk.
Hezbollah isn't Israel's only concern. Israeli officials believe it is only a matter of time before Assad's government collapse, and they fear that some of the Islamic extremist groups battling him will turn their attention toward Israel once Assad is gone.
Reflecting Israel's anxiety, the Israeli military called up several thousand reservists earlier this week for what it called a "surprise" military exercise on its border with Lebanon.
Obama has said the use of chemical weapons would have "enormous consequences," but has also said he needs more definitive proof before making a decision about how to respond.
Obama said Friday that he didn't foresee a scenario in which the U.S. would send troops to Syria. Instead, Defense Secretary Chuck Hagel has said Washington is reviewing its opposition to arming the opposition.
The U.S. so far has balked at sending weapons to the rebels, fearing the arms could end up in the hands of al-Qaida-linked groups or other extremists in the opposition ranks.
Secretary of State John Kerry, meanwhile, is heading to Moscow next week to try to persuade Russian President Vladimir Putin to support, or at least not veto, a fresh effort to impose U.N. penalties on Syria if Assad doesn't begin political transition talks with the opposition.
Russia, alongside China, has blocked U.S.-led efforts three times at the United Nations to pressure Assad into stepping down.
In Syria, about 4,000 Sunni Muslims fled the coastal town of Banias on Saturday, a day after reports circulated that dozens of people, including children, had been killed by pro-government gunmen in the area, activists said.
Also Saturday, Assad made his second public appearance in a week in the capital Damascus. Syrian state TV said Assad, who rarely appears in public, visited a Damascus campus, and footage showed him being thronged by a large crowd. The report said Assad inaugurated a statue dedicated to "martyrs" from Syrian universities who died in the country's uprising and civil war.
Federman reported from Jerusalem. Associated Press writers Ian Deitch in Jerusalem, Bassem Mroue in Beirut, and Bradley S. Klapper and Josh Lederman in Washington contributed reporting.
Obama says he won't comment on Israeli airstrike
San Jose, Costa Rica • President Barack Obama says he won't comment on an Israeli airstrike against Syria that targeted a shipment of advanced missiles believed to be headed for the Lebanese militant group Hezbollah.
Israeli officials on Saturday confirmed the strike, which took place early Friday.
Obama told the Spanish-language network Telemundo in an interview that he will defer to the Israeli government for comment. He also repeated his view that the Israelis justifiably have to guard against the transfer of advanced weapons to organizations like Hezbollah. The U.S. considers Hezbollah a terrorist organization.
Obama conducted the interview Saturday, and a portion of the president's answer to a question about the airstrike was broadcast on MSNBC.