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The American officials, who spoke on the condition of anonymity, said Israel had notified the United States about the attack, which the Syrian government condemned as an act of "arrogance and aggression." Israel's move demonstrated its determination to ensure that Hezbollah — its arch foe in the north — is unable to take advantage of the chaos in Syria to bolster its arsenal significantly.

The predawn strike was the first time in more than five years that Israel's air force had attacked a target in Syria. While there was no expectation that the beleaguered Assad government had an interest in retaliating, the strike raised concerns that the Syrian civil war had continued to spread beyond its border.

In a statement, the Syrian military denied that a convoy had been struck. It said the attack had hit a scientific research facility in the Damascus suburbs that was used to improve Syria's defenses, and called the attack "a flagrant breach of Syrian sovereignty and airspace."

Israeli officials would not confirm the airstrike, a common tactic here. But it came after days of intense security consultations with Prime Minister Benjamin Netanyahu regarding the possible movement of chemical and other weapons around Syria, and warnings that Jerusalem would take action to thwart any possible transfers to Hezbollah.

Thousands of Israelis have crowded gas-mask distribution centers over the last two days. On Sunday, Israel deployed its Iron Dome missile defense system in the north, near Haifa, which was heavily bombed during the 2006 war with Lebanon.

Syria and Israel are technically in a state of war but have long maintained an uneasy peace along their decades-old armistice line. Israel has mostly watched warily and tried to stay out of Syria's raging civil war, fearful of provoking a wider confrontation with Iran and Hezbollah. In November, however, after several mortars fell on Israel's side of the border, its tanks struck a Syrian artillery unit.

Several analysts said that despite the increased tensions, they thought the likelihood of retaliation for the airstrike was relatively low.

"It is necessary and correct to prepare for deterioration — that scenario exists," Danny Yatom, a former chief of the Mossad, Israel's intelligence agency, told Ynet, a news Web site. "But in my assessment, there will not be a reaction, because neither Hezbollah nor the Syrians have an interest in retaliating."

Syria's president, Bashar al-Assad, "is deep in his own troubles," Mr. Yatom said, "and Hezbollah is making a great effort to assist him, in parallel with its efforts to obtain weapons, so they won't want to broaden the circle of fighting."

In the United States, the State Department and Defense Department would not comment on reports of the strike.

Russia, which has carried out a vigorous diplomatic battle to deter foreign military intervention in the Syrian conflict for more than a year, issued a statement of concern early on Thursday, describing the strikes as "an attack by Israel's air force on objects in Syria, near Damascus."

"If this information is confirmed, this is an unprovoked attack on the territory of a sovereign nation, which blatantly violates the U.N. charter and is unacceptable and unjustified whatever its motives," said a statement posted on the web site of the Russian Foreign Ministry.

Moscow said it would take immediate steps to clarify what had happened, and reiterated its longstanding insistence on a political solution and "the unacceptability of any kind of external intervention."

The episode illustrated how the escalating violence in Syria, which has already killed more than 60,000, is drawing in neighboring states and threatening to destabilize the region further.

Iran has firmly allied itself with Mr. Assad, sending personnel from its Islamic Revolutionary Guards Quds Force to Syria and ferrying military equipment to Syria through Iraqi airspace.

Hezbollah, which plays a decisive role in Lebanese politics and has supported Mr. Assad during the uprising by providing training and logistical support to his forces, has long relied on Syria as both a source of weapons and a conduit for weapons flowing from Iran. Some analysts think Hezbollah may be trying to stock up on weapons in case Mr. Assad falls and is replaced by a leadership that is hostile to the militia.

One American official said the trucks targeted on Wednesday were believed to have been carrying sophisticated SA-17 antiaircraft weapons. Hezbollah's possession of such weapons would be a serious worry for the Israeli government, said Matthew Levitt, a former intelligence official who is at the Washington Institute for Near East Policy.

"Israel is able to fly reconnaissance flights over Lebanon with impunity right now," Mr. Levitt said. "This could cut into its ability to conduct aerial intelligence. The passing along of weapons to Hezbollah by the regime is a real concern."

While some analysts said the Assad government might be providing the weapons to Hezbollah as a reward for its support, others were skeptical that Syria would relinquish such a sophisticated system.

Hezbollah has boasted that it has replenished and increased its weapons stocks since the 2006 war with Israel. During that war, Israeli bombardments destroyed some of its arms, and other missiles were used in a barrage that killed Israelis as far south as Haifa and that drove residents of northern Israel into shelters.

The Syrian statement, carried by state television, said an unidentified number of Israeli jets flying below radar had hit the research facility in the Jimraya district, killing two people and causing "huge material damage." It cast the attack as "another addition to the history of Israeli occupation, aggression and criminality against Arabs and Muslims."

"The Syrian government points out to the international community that this Israeli arrogance and aggression is dangerous for Syrian sovereignty," the statement said, "and stresses that such criminal acts will not weaken Syria's role nor will discourage Syrians from continuing to support resistance movements and just Arab causes, particularly the Palestinian issue."

The Lebanese Army said in a statement on Wednesday that Israeli warplanes had carried out two sorties, circling over Lebanon for hours on Tuesday and before dawn on Wednesday, but made no mention of any attacks.

Israel has long maintained a policy of silence on pre-emptive military strikes. In October, officials refused to discuss an accusation by Sudan that Israeli airstrikes had destroyed a weapons factory in Khartoum, its capital. Israel also never admitted to the bombing of a Syrian nuclear reactor in 2007; Syria kept mum about that attack, too, and the ambiguity allowed the event to pass without Damascus feeling pressure to retaliate.

Amnon Sofrin, a retired brigadier general and former Israeli intelligence officer, told reporters here on Wednesday that Hezbollah, which is known to have been storing some of its more advanced weapons in Syria, was now eager to move everything it could to Lebanon. He said Israel was carefully watching for convoys transferring weapons systems from Syria to Lebanon.

Israel has made it clear that if the Syrian government loses control over its chemical weapons or transfers them to Hezbollah, Israel will feel compelled to act. Avi Dichter, the minister for the home front, told Israel Radio on Tuesday that options to prevent Syria from using or transferring the weapons included deterrence and "attempts to hit the stockpiles."

"Everything will have ramifications," Mr. Dichter said. "The stockpiles are not always in places where operative thinking is possible. It could be that hitting the stockpiles will also mean hitting people. Israel has no intention of hitting residents of Syria."