* Strike said to have hit building; Israel not commenting
* Israel worries Hezbollah, jihadis could seize Assad's arms
Israeli warplanes have carried out an air strike in Syria, a security source in the region said on Saturday, confirming a disclosure by a U.S. official.
The attack took place on Friday after Prime Minister Benjamin Netanyahu's security cabinet approved it in a secret meeting on Thursday night, the source told Reuters on condition of anonymity.
The target was not a Syrian chemical weapons facility, the source said.
Israeli officials have declined all comment on the event, which was reported by American media on Friday.
The Jewish state had previously made clear it was poised to resort to force to prevent advanced Syrian weapons, including President Bashar al-Assad's reputed chemical arsenal, reaching Islamist rebels fighting to topple him or his Hezbollah guerrilla allies in Lebanon.
A U.S. official, who also declined to be identified, told Reuters on Friday the air strike apparently targeted a building.
CNN quoted unnamed U.S. officials as saying Israel most likely conducted the strike "in the Thursday-Friday time frame" and its jets did not enter Syrian air space.
The Israeli air force has so-called "standoff" bombs that coast dozens of kilometres (miles) across ground to their targets once fired. That could, in theory, allow Israel to attack Syria from its own turf or from neighbouring Lebanon.
The CNN report said that during the time frame of the attack, the United States had collected information showing Israeli warplanes overflying Lebanon.
Bashar Ja'afari, the Syrian ambassador to the United Nations, told Reuters: "I'm not aware of any attack right now."
In January this year, Israel bombed a convoy in Syria, apparently hitting weapons destined for Hezbollah, according to diplomats, Syrian rebels and security sources in the region.
Israel has not formally confirmed carrying out that strike.
Hezbollah fought a 34-day war with Israel in 2006.
Israeli remains technically at war with neighbouring Syria. It captured Syria's Golan Heights in the 1967 Middle East war, built settlements and annexed the land. Yet belligerence was rare and the borderland has remained largely quiet for decades.
But Israeli concerns have risen since Islamist fighters linked to al-Qaeda assumed a prominent role in the armed insurrection against Assad. Israelis believe one in 10 of the rebels is a jihadi who might turn his gun on them once Assad is gone. They also worry that Hezbollah guerrillas allied to Assad could obtain his chemical arsenal and other advanced weaponry.