JERUSALEM -- Israeli warplanes carried out a strike deep inside Syrian territory Wednesday, U.S. officials reported, saying they believed that the target was a convoy carrying sophisticated anti-aircraft weaponry on the outskirts of Damascus that was intended for the Hezbollah Shiite militia in Lebanon.
The U.S. officials, who spoke on 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 Tel Aviv's 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 in Jerusalem. But it came after days of intense security consultations with Prime Minister Benjamin Netanyahu regarding possible movement of chemical and other weapons around Syria, and warnings that Israel 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 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 website. "But in my assessment, there will not be a reaction, because neither Hezbollah nor the Syrians have an interest in retaliating." Syrian President Bashar 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.
The episode illustrated how the escalating Syrian violence, 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 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 throughout the uprising, 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 now, in case Mr. Assad falls and is replaced by a leadership hostile to the Shiite militia.
One U.S. official said the trucks targeted Wednesday were believed to have been carrying sophisticated SA-17 anti-aircraft 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 when it might need it at home.
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 Lebanese army said in a statement Wednesday that Israeli warplanes had carried out two sorties, circling over Lebanon for hours Tuesday and before dawn Wednesday, but made no mention of any attacks.
Jerusalem has long maintained a policy of silence on pre-emptive military strikes. Israel also never admitted to the bombing of a Syrian nuclear reactor in 2007; Syria kept mum about that attack, too.