Syria has accused Israel of launching rocket attacks on Damascus, after a night of huge explosions near the city.
Syrian state media said the rockets hit the Jamraya research centre, which Western officials have suggested is involved in chemical weapons research.
Israeli radio quoted a senior security official confirming an attack, and sources said it targeted weapons bound for Hezbollah militants in Lebanon.
It is the second suspected Israeli strike in Syria in two days.
On Friday Israeli aircraft hit a shipment of missiles near the Lebanon border, according to unnamed US and Israeli officials.
The BBC's Yolande Knell in Jerusalem says the latest developments are a significant escalation in Israel's involvement in the conflict.
She says Israel has already responded to fears of retaliation by locating two batteries of its Iron Dome missile defence system near Haifa, close to the Lebanese border.
Damascus was shaken by repeated explosions coming from the north-western suburbs.
Amateur video footage and eye witness testimony suggested rocket attacks had hit weapons dumps, triggering dramatic orange-flamed blasts.
The area houses numerous military facilities, including the Jamraya research centre, designated by Syria as a scientific research centre "in charge of raising our level of resistance and self-defence".
A state TV bulletin said: "The new Israeli attack is an attempt to raise the morale of the terrorist groups, which have been reeling from strikes by our noble army."
Damascus-based journalist Alaa Ebrahim told the BBC it was "the biggest explosion" the city had seen since the conflict began two years ago.
He said residents living near Jamraya reported feeling a "mild earthquake" just before the blast, indicating that the rockets may have hit an underground facility.
He added that the Syrian army was likely to have suffered major casualties in the attack.
The UK-based Syrian Observatory for Human Rights quoted eyewitnesses in the area as saying they saw jets in the sky at the time of the explosions.
The Jamraya facility was also apparently hit in an Israeli air strike in January.
Israeli officials confirmed the January strike, but insisted it had targeted trucks carrying missiles to Hezbollah.
After the latest attack, unnamed Western intelligence sources have again said the target was a weapons cache heading for Lebanon.
Israel has repeatedly said it would act if it felt advanced weapons were being transferred to militant groups in the region, especially Hezbollah.
Analysts say the air strikes are unlikely to have a major effect on the civil war in Syria.
The latest reports from coastal regions around the town of Baniyas suggest dozens of Sunnis have been massacred in a campaign of sectarian cleansing.
The government said it had pushed back "terrorist groups" and restored security to the area.
The US said it was "appalled by the horrific reports" but that it did not foresee sending US troops to Syria.
However, the US is no longer ruling out supplying weapons to the rebels.
More than 70,000 people are estimated to have been killed since the conflict erupted in March 2011.