Israeli warplanes attacked a shipment of Russian missiles inside a Syrian government stronghold, officials said yesterday.
It was a development that threatened to add another volatile layer to regional tensions from the Syrian civil war.
The revelation came as the government of President Bashar Assad met a key deadline in an ambitious plan to eliminate Syria's entire chemical weapons stockpile by mid-2014 and avoid international military action.
The announcement by a global chemical weapons watchdog that the country has completed the destruction of equipment used to produce the deadly agents highlights Assad's willingness to co-operate, and puts more pressure on the divided and outgunned rebels to attend a planned peace conference.
An Obama administration official confirmed the Israeli airstrike, but provided no details. Another security official said the attack occurred late on Wednesday in the Syrian port city of Latakia and that the target was Russian-made SA-125 missiles.
There was no immediate confirmation from Syria.
Since the civil war in Syria began in March 2011, Israel has carefully avoided taking sides, but has struck shipments of missiles inside Syria at least twice this year.
The Syrian military, overstretched by the civil war, has not retaliated, and it was not clear whether the embattled Syrian leader would choose to take action this time.
Assad may decide to again let the Israeli attack slide, particularly when his army has the upper hand on the battlefield inside Syria.
Israel has repeatedly declared a series of red lines that could trigger Israeli military intervention, including the delivery of "game-changing" weapons to the Syrian-backed Lebanese Hezbollah group.
Israel has never officially confirmed taking action inside Syria to avoid embarrassing Assad and sparking a potential response.
But foreign officials say it has done so several times when Israeli intelligence discovered that sophisticated missiles were on the move.
In January, an Israeli airstrike in Syria destroyed a shipment of advanced anti-aircraft missiles bound for Hezbollah, according to US officials.
And in May, it was said to have acted again, taking out a shipment of Iranian-made Fateh-110 missiles at a Damascus airport.
Syrian activists and opposition groups reported strong explosions on Wednesday night that appeared to come from inside an air defence facility in Latakia. They said the cause of the blasts was not known.
The announcement yesterday that Syria had completed the destruction of equipment used to produce chemical weapons came a day before a November 1 deadline set by the Hague-based watchdog, the Organisation for the Prohibition of Chemical Weapons (OPCW).
But while some experts portrayed the step as a milestone, others said it has little impact as long as Syria still has its entire remaining stockpile of functioning chemical weapons.
"Only after those weapons have been destroyed or removed from Syrian control will the state be demilitarised," said David Reeths, director at HIS Jane's Consulting.
With the initial stage of verification and destruction of weapons machinery completed, the hard task now begins.
The executive committee of the OPCW has until November 15 to decide how best permanently to destroy Syria's chemical weapons programme and its stockpile of deadly mustard gas, sarin and precursor chemicals.
Assad has so far met all required deadlines according to the strict timeline, demonstrating his willingness to go to great lengths to avoid international military action.
The US-Russian deal to destroy Syria's stockpile averted a US military strike against the Syrian government that appeared certain in August, following a chemical weapons attack near Damascus that killed hundreds, and which the US blamed on Assad.
By making him a partner in implementing the disarmament deal, the agreement appears to have restored some of Assad's legitimacy while angering his opponents, who now balk at attending political transition talks the US hopes will begin in Geneva this month.
No final date has been set for the talks, and there have been disagreements among opposition groups on whether to attend or not, and the conditions for taking part.
At a Senate hearing in Washington yesterday, Senator John McCain said Assad, who was about to be toppled a year ago, has "turned the tide" while continuing to slaughter innocent civilians.
Fighting continued across many parts of the country, including in the town of Safira, in northern Aleppo province. Experts say the town is home to a chemical weapons production facility, as well as storage sites.
Activists said troops were advancing yesterday in the town, capturing several neighbourhoods and causing casualties on both sides.
Also yesterday, the Syrian Observatory for Human Rights, a Britain-based Syria watchdog, said more than 120,000 people have been killed since the start of the country's conflict nearly three years ago.
In July, the UN estimated 100,000 have died in the conflict since March 2011. It has not updated that figure since.