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 cooperate, and puts more pressure on the divided and outgunned rebels to attend a planned peace conference.
An Obama administration official confirmed the Israeli airstrike overnight, but provided no details. Another security official said the attack occurred late Wednesday in the Syrian port city of Latakia and that the target was Russian-made SA-125 missiles.
The officials spoke on condition of anonymity because they were not authorized to publicly discuss the attack. 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 determined 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 U.S. 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.
The Fateh-110s have advanced guidance systems that allow them to travel up to 200 miles (300 kilometers) per hour with great precision. Their solid-fuel propellant allows them to be launched at short notice, making them hard to detect and neutralize.
Israel has identified several other weapons systems as game changers, including chemical weapons, Russian-made Yakhont missiles that can be fired from land and destroy ships at sea, and Russian SA-17 anti-aircraft missiles. Israel's January airstrike is believed to have destroyed a shipment of SA-17s.
Syrian activists and opposition groups reported strong explosions Wednesday night that appeared to come from inside an air defense facility in Latakia. They said the cause of the blasts was not known.
The announcement Thursday that Syria had completed the destruction of equipment used to produce chemical weapons came one day ahead of a Nov. 1 deadline set by the Hague-based watchdog, the Organization for the Prohibition of Chemical Weapons.
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 demilitarized," 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 Organization for the Prohibition of Chemical Weapons has until Nov. 15 to decide how best to permanently destroy Syria's chemical weapons program and its stockpile of deadly mustard gas, sarin and precursor chemicals.
It's not yet clear how and where the arsenal will be destroyed, but carrying out the work in Syria or transporting the chemical weapons out of the country for destruction elsewhere are both fraught with risks amid the ongoing civil war. The country is believed to have around 1,000 metric tons of chemical weapons.
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.
"This is a clear indication of the Syrian government's wish to cooperate and abide by its commitments," said Syrian lawmaker Issam Khalil. He said Syria knows "full well that the U.S. has not ceased its hostile policies toward Syria and will attempt to exploit any excuse — however small and inconsequential — to carry out a military strike against Syria."
The U.S.-Russian deal to destroy Syria's stockpile averted a U.S. military strike against the Syrian government that appeared certain in August, following a chemical weapons attack near Damascus that killed hundreds the U.S. 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 U.S. hopes will begin in Geneva in November.
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. Syria's main opposition group in exile, the Syrian National Coalition, postponed its general council meeting in Istanbul from Friday to Nov. 10, pending further discussions on the highly divisive talks.
U.N.-Arab League envoy Lakhdar Brahimi, currently in Damascus, has urged both sides to come to the talks without preconditions. But both have placed seemingly unrealistic conditions for attending.
At a Senate hearing in Washington on Thursday, Sen. 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 at a high pace 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 Thursday in the town, capturing several neighborhoods and causing casualties on both sides.
Also on Thursday, 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 U.N. estimated 100,000 have died in the conflict since March 2011. It has not updated that figure since.
The violence underscored the dangers the chemical weapons' inspectors face as they race against tight deadlines in the midst of an ongoing civil war.
Earlier this week, the inspectors said they had completed their first round of verification work, visiting 21 of 23 sites declared by Damascus. They were unable to visit two sites because of security concerns, the inspectors said.
On Thursday, the chemical weapons agency said the two locations were, according to Syria, "abandoned and ... the chemical weapons program items they contained were moved to other declared sites, which were inspected."
It was not immediately clear if the facility in Safira was one of the two sites.
Commenting on the two sites, OPCW spokesman Michael Luhan said, "it was just deemed too risky." He told the AP that Syrian authorities were not able to offer the necessary security guarantees for inspectors to visit those sites.
He added, however, that the Syrian side provided "quite compelling documentary evidence" that equipment in one of the sites was moved to another location that inspectors did visit.
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