Israel launched a second wave of strikes on targets inside Syria early on Saturday, with huge explosions reported in Damascus at the site of a suspected chemical weapons facility.
Syrian state media reported Israeli missiles struck the Jamraya military research centre on the outskirts of Damascus in the early hours of Sunday.
Israel has declined to comment on the latest attack, but an unnamed intelligence source said the target was "stores of Fateh-110 missiles that were in transit from Iran to Hezbollah".
Israeli leader Binyamin Netanyahu has repeatedly threatened his forces will target weapons believed to be bound for Hezbollah militants in Lebanon.
The Jamraya building was the target of an earlier Israeli strike in January.
Video footage posted online this morning showed a huge explosion in the Syrian capital.
Earlier in the day, Israeli officials in Washington confirmed that a series of strikes took place the previous day, which they said had been aimed at a convoy carrying weapons across the border into Lebanon.
The Syrian regime said the attacks showed Israel was now actively supporting anti-government forces in the country's civil war, and that its interventions were aimed at boosting rebel groups.
"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," state broadcasts said.
Witnesses said the explosions last night were the biggest since the onset of the conflict two years ago, and marked a dramatic escalation.
One witness, Syrian journalist Alaa Ebrahim described the attack as like "a mild earthquake", adding that it was the " biggest explosion" he had seen since the conflict began.
Online posts showed footage of the blasts at Jamraya, a site designated by the government as a scientific research centre "in charge of raising our level of resistance and self-defence".
Witnesses reported seeing jets in the sky at the time of the explosions, according to the UK-based Syrian Observatory for Human Rights. Other sources indicated an underground facility had been hit.
An intelligence official in the Middle East, who spoke on condition of anonymity, confirmed Israel launched airstrikes in Damascus early Sunday.
"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.
Hezbollah's al-Manar TV said a weapons facility was targeted by the strike, and quoted unnamed Syrian security officials saying three sites including military barracks, arms depots and an air defence center were hit.
One of the strikes was aimed at a military position in the village of Saboura, west of Damascus and about 10 km from the Lebanon border, Lebanon's Al-Mayadeen TV reported.
An amateur video, thought to be genuine, showed fire lighting up the night skies in Damascus.
Uzi Rubin, a former Israeli defence ministry official, said Syria's Fateh-110 missiles would leave all Israel within range of Hezbollah attacks.
"If fired from southern Lebanon they can reach Tel Aviv and even (the southern city of) Beersheba." He said the rockets are five times more accurate than the scud missiles Hezbollah had fired in the past. "It is a game-changer because they are a threat to Israel's infrastructure and military installations," he said.
Former Israeli Defence Minister Shaul Mofaz said Israel feared weapons falling into Hezbollah's hands.
"We must remember that the Syrian system is falling apart and Iran and Hezbollah are involved up to their necks in Syria helping Bashar Assad," he told Israel Radio.
"There are dangers of weapons trickling to the Hezbollah and chemical weapons trickling to irresponsible groups like al-Qaida."
In separate developments inside Syria, hundreds fled coastal areas where activists said government troops massacred around 200 people, many of them women and children.
Video footage also showed the bodies of 10 people said to have been killed in Ras al Nabaa, in the city of Banias, in an attack overnight. At least 60 people were feared dead.