In what is believed to be Israel's second strike on the troubled country in three days, Syrian TV said rockets launched in the early hours hit sites including a military research centre at Jamraya, northwest of the capital, about 15km from the border with Lebanon, where Hezbollah is based.

Israel and the US refused to publicly confirm Israeli involvement in the latest attack on the regime of President Bashar al-Assad, whose war on rebels has killed an estimated 70,000 people since March 2011. A day earlier, Washington sources confirmed that Israel had hit a storage depot for missiles, also believed to be on the way to Hezbollah, on Friday.

The target of all the strikes was Fateh-110 missiles, which have precise guidance systems with better aim than anything Hezbollah has in its arsenal, an official told The Associated Press.

"We are under very strict instructions not to confirm anything," one Israeli official told The Australian last night.

But news agency AFP quoted an Israeli official saying it was carried out by Israel and that "any time Israel learns about the transfer of weapons from Syria to Lebanon it will attack".

The Institute for National Security Studies, which has close ties to the government, also suggested the attack was by Israel.

The director of the INSS, former Israeli military intelligence chief Amos Yadlin, told Israeli Army Radio: "Iran is testing the determination both of Israel and the US regarding red lines, and what it sees in Syria is that at least some of the players take the red lines seriously."

Israeli legislator Shaul Mofaz, a former defence minister and a former chief of staff, declined to confirm the airstrike but said Israel feared weapons falling into the hands of the Islamic militant group amid the chaos in Syria.

"We must remember 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. The BBC quoted a Damascus-based journalist as saying it was the biggest blast in the capital since the Arab Spring-inspired civil war began. He said the army was likely to have suffered major casualties.

Residents said they felt "a mild earthquake" just before the explosion, suggesting the rockets may have hit an underground facility, BBC reported.

The latest incident comes only days after US Defence Secretary Chuck Hagel said Washington was re-thinking its opposition to providing arms to Syria's rebels.

Washington is considering how to respond to indications that the Syrian regime may have used chemical weapons in its civil war. President Barack Obama has described the use of such weapons as a "red line", and the administration is weighing its options - including possible military action.

Israel has said it wants to stay out of the brutal Syria war, but Prime Minister Benjamin Netanyahu has repeatedly stated the Jewish state would be prepared to take military action to prevent sophisticated weapons from flowing from Syria to Hezbollah or other extremist groups.

Israel and Hezbollah fought a month-long war in mid-2006 that ended in a stalemate.

If Israel was involved in the latest attack at Jamraya it would be the third intervention this year.

In January a convoy with weapons leaving Jamraya was attacked from the air - Israel's then defence minister, Ehud Barak, confirmed it was an Israeli operation, and proof that "when we say something we mean it".

Mr Obama has said Israel is justified in trying to prevent weapons being transported to Hezbollah in neighbouring Lebanon.

The Assad regime is using reports of Israeli involvement to try to curb support for the rebels.

The state-run Sana TV network last night said: "The new Israeli aggression is a clear attempt to alleviate the pressure on the armed terrorist groups after our army beat them back in several regions and after the army's victories on the road to recovering security and stability in Syria."