The officials say the shipment was not of chemical arms, but of “game changing” weapons bound for the Lebanese Hezbollah group.
They say the airstrike was early Friday.
They did not say where it took place.
They spoke Saturday on condition of anonymity because they were discussing a secret military issue.
Earlier on Friday, anonymous US officials reported the airstrike.
It did not appear that a chemical weapons site was targeted, the US officials said, and one official said the strike appeared to have hit a warehouse.
Earlier this week, Hezbollah leader Sheikh Hassan Nasrallah said his group would assist Syrian President Bashar Assad if needed in the effort to put down a 2-year-old uprising.
Israeli embassy spokesman Aaron Sagui would not comment on Friday night specifically on the report of an Israeli strike into Syria.
“What we can say is that Israel is determined to prevent the transfer of chemical weapons or other game-changing weaponry by the Syrian regime to terrorists, specially to Hezbollah in Lebanon,” Sagui said in an email to the AP.
Syrian UN Ambassador Bashar Ja’afari told Reuters: “I’m not aware of any attack right now.”
It was not immediately clear where the airstrike took place, or whether the air force carried out the strike from Lebanese or Syrian airspace.
The Israeli air force has so-called “standoff” bombs that coast dozens of kilometres across ground to their targets once fired.
That could, in theory, allow Israel to attack Syria from its own turf or from neighbouring Lebanon.
Lebanese authorities reported unusual intensive Israeli air force activity over their territory on Thursday and Friday.
A Lebanese security source said his initial impression was that Israeli overflights were monitoring potential arms shipments between Syria and Lebanon, potentially to Hezbollah.
“We believe that it is linked to Israel’s concerns over the transfer of weapons, particularly chemical weapons, from Syria to its allies Lebanon,” said the official, who asked not to be named.
Syrian opposition sources, usually quick to announce rumours of Israeli air strikes, said they had not heard of an attack on Syrian territory.
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.
In 2007, Israeli jets bombed a suspected nuclear reactor site along the Euphrates River in northeastern Syria, an attack that embarrassed and jolted the Assad regime and led to a buildup of the Syrian air defense system.
The Israeli strike also follows days of renewed concerns that Syria might be using chemical weapons against opposition forces.
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