WASHINGTON -- The airstrike that Israeli warplanes carried out in Syria was directed at a shipment of advanced surface-to-surface missiles from Iran that Israel believed was intended for Hezbollah, the militant Lebanese organization, American officials said Saturday.
It was the second time in four months that Israel had carried out an attack in foreign territory intended to disrupt the pipeline of weapons from Iran to Hezbollah, and the raid was a vivid example of how regional adversaries are looking after their own interests as Syria becomes more chaotic.
Iran and Hezbollah have both backed President Bashar al-Assad in the Syrian civil war, now in its third year. But as fighting in Syria escalates, they also have a powerful stake in expediting the delivery of advanced weapons to Hezbollah in case Mr. Assad loses his grip on power.
Israel, for its part, has repeatedly cautioned that it will not allow Hezbollah to receive "game changing" weapons that could threaten the Israeli heartland after a post-Assad government took power.
And as Washington considers how to handle evidence of chemical weapons use by the Syrian government, a development it has described as a "red line," Israel is clearly showing that it will stand behind the red lines it sets.
"The Israelis are saying, 'O.K., whichever way the civil war is going, we are going to keep our red lines, which are different from Obama's,' " said Ehud Yaari, an Israel-based fellow of the Washington Institute for Near East Policy.
The missiles that were the target of the raid had been sent to Syria by Iran and were being stored in a warehouse at Damascus International Airport when they were struck, according to an American official.
Two prominent Israeli defense analysts said military officials had told them that the targeted shipment included Scud Ds, which Syrians have developed from Russian weapons and have a range up to 422 miles -- long enough to reach Eilat, in southernmost Israel, from Lebanon.
But an American official, who asked not to be identified because he was discussing intelligence reports, said they were Fateh-110s.
The Fateh-110 is a mobile, accurate, solid-fueled missile that represents a considerable improvement over the liquid-fueled Scud missile. American officials have said it has the range to strike Tel Aviv and much of Israel from southern Lebanon.
A Pentagon official said in 2010 that Hezbollah was believed to already have a small supply of Fateh-110s. Additional missiles could increase Iran's ability to threaten Israel through its Lebanese proxy if Israel ever mounted airstrikes against Iran's nuclear installations.
Syrians with knowledge of security and military matters confirmed the strike, which took place overnight Thursday, saying that Iran had sent arms and rockets to the airport intending to resend them to Hezbollah.
Israeli officials have declined to publicly discuss the operation. But Israel has repeatedly said it is prepared to take military action to stop the shipment of advanced arms or chemical weapons to Hezbollah.
Syrian forces loyal to Mr. Assad have used Fateh-110 missiles against the Syrian opposition. Some American officials are unsure whether the new shipment was intended for use by Hezbollah or by the Assad government, which is believed to be running low on missiles in its bloody civil war.
But one American official said the warehouse that was struck in the Israeli attack was believed to be under the control of operatives from Hezbollah and Iran's paramilitary Quds force.
In carrying out the raid, Israeli warplanes did not fly over the Damascus airport. Instead, they fired air-to-ground weapons, apparently using the airspace of neighboring Lebanon.
The Lebanese Army said in a statement that Israeli military aircraft "violated the Lebanese airport" on Thursday night and early Friday morning and were flying in circles over several areas of the country.
A spokesman for the Israeli Embassy in Washington declined on Friday night to comment on the airstrike, saying only in a statement, "Israel is determined to prevent the transfer of chemical weapons or other game-changing weaponry by the Syrian regime to terrorists, especially to Hezbollah in Lebanon."
In late January, Israel carried out similar airstrikes in Syria against a convoy carrying SA-17 antiaircraft weapons. The transfer of those weapons to Hezbollah would have jeopardized the Israeli Air Force's ability to operate in Lebanese airspace.
Israeli officials have also refused to publicly confirm the January attack.
Israel's official silence reveals the broader dilemma it faces in how to handle Syria's upheaval. After 40 years of quiet on its northeastern border, Israel is now deeply worried about violence spilling over into its territory and about a post-Assad Syria being a vast, ungoverned area controlled by Islamist or jihadist groups, with no central authority to control militant activity.
But leaders in Jerusalem believe that they have few options beyond the targeted attacks on convoys or warehouses to affect the situation in Syria, seeing any direct action by Israel as likely to backfire by bolstering or uniting anti-Israel forces.
Jonathan Spyer, an expert on Syria and Hezbollah at the Interdisciplinary Center in Herzliya, Israel, called Thursday's strike "extremely significant," and predicted more such attempts to transfer weapons -- and Israeli efforts to stop them -- in the coming weeks and months.
"Clearly Hezbollah is hoping to benefit from its engagement in Syria, and clearly Israel is committed to preventing that," he said. Mr. Spyer said that in striking the warehouse, Israel was taking a "calculated risk" that its limited intervention would provoke a limited response, if any.
The Israeli attack came days after Hezbollah's leader, Hassan Nasrallah, issued some of his strongest statements yet of support for Mr. Assad, edging closer to confirming what the Obama administration has already reported: that Hezbollah is backing him militarily, not merely tolerating border crossings by some of its members to defend Lebanese citizens in Syria, as Hezbollah has long maintained.
Mr. Nasrallah said Hezbollah -- using the word "we" -- would not allow Syria to fall to an armed assault that he said was backed by America and Israel, and added that the party was defending civilians of all sects in Qusayr, a city in Homs Province near the Lebanese border, where rebels say Hezbollah has led recent battles against them.
Michael R. Gordon reported from Washington, and Jodi Rudoren from Jerusalem. Reporting was contributed by Eric Schmitt and David E. Sanger from Washington; Anne Barnard from Beirut, Lebanon; and an employee of The New York Times from Damascus, Syria.
This article originally appeared in The New York Times.