Israel carried out its second air strike in days on Syria early on Sunday, a Western intelligence source said, in an attack that shook Damascus with a series of powerful blasts and drove columns of fire into the night sky.
Israel declined to comment, but Syria accused the Jewish state of carrying out a raid on a military facility just north of the capital.
The explosions came soon after an Israeli official said his country had carried out an air strike earlier in the week targeting missiles in Syria intended for the Lebanese militant group Hezbollah.
The target of Sunday's attack, according to Syrian media, was the same Jamraya military research centre which was hit by Israel in another assault in January. Jamraya, on the northern approaches to Damascus, is just 15 km (10 miles) from the Lebanese border.
Video footage uploaded onto the Internet by activists showed a series of explosions. One lit up the skyline over the city, while another sent up a tower of flames and secondary blasts.
The Western intelligence source told Reuters Israel carried out the attack and the operation hit Iranian-supplied missiles which were en route to Hezbollah.
"In last night's attack, as in the previous one, what was attacked were stores of Fateh-110 missiles that were in transit from Iran to Hezbollah," the source said.
The British-based Syrian Observatory for Human Rights said the scale of the attack meant it was beyond the military capability of Syrian rebels, and quoted eyewitnesses in the area as saying they saw jets in the sky at the time of the blasts.
The Observatory said the blasts hit Jamraya as well as a nearby ammunition depot. Other activists said a missile brigade and two Republican Guard battalions may also have been targeted in the heavily militarised area just north of Damascus.
Syria's state television said the strikes were a response to recent military gains by President Bashar al-Assad's forces against rebels.
"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," it said.
Reports by activists and state media are difficult to verify in Syria because of restrictions on journalists operating there.
If confirmed, Sunday's attack would be Israel's third strike inside Syria since late January, but there was no immediate comment from Israeli officials. "We don't respond to this kind of report," an Israeli military spokeswoman told Reuters.
Missile "better than scud"
Israel has repeatedly made clear it is prepared to use force to prevent advanced weapons from Syria reaching Lebanon's Shi'ite Muslim Hezbollah guerrillas, who fought a 34-day war with Israel in 2006. Assad and Hezbollah are allied to Iran, Israel's arch-enemy.
Uzi Rubin, an Israeli missile expert and former defence official said the Fateh-110 missile "is better than the Scud, it has a half-ton warhead". Iran has said it adapted the missile for anti-ship use by installing a guidance system, he added.
With Assad battling a more than 2-year-old insurgency, the Israelis also worry that the majority Sunni Islamist rebels could loot his arsenals and eventually hit the Jewish state, ending four decades of relative cross-border calm.
The US state department and Pentagon had no immediate comment and the Israeli Embassy in Washington declined comment. Speaking shortly before Sunday's reported attack, President Barack Obama said Israel had a right to act.
"The Israelis justifiably have to guard against the transfer of advanced weaponry to terrorist organisations like Hezbollah," he told Telemundo network during a tour of Latin America.
There was no immediate indication of how Syria would respond to Sunday's attack. After Israel's January raid Damascus protested to the United Nations and the Syrian ambassador to Lebanon promised a "surprise decision", but no direct military retaliation followed.
The uprising against Assad began with mainly peaceful protests that were met with force and grew into a bloody civil war in which the United Nations says at least 70,000 people have been killed.
Assad has lost control of large areas of north and eastern Syria, and is battling rebels on the fringes of Damascus.
But his forces have launched counter-offensives in recent weeks against the mainly Sunni Muslim rebels around the capital and near the city of Homs, which links Damascus with the Mediterranean heartland of Assad's minority Alawite sect.