The dead were all foreign paramilitaries fighting alongside President Bashar al-Assad's forces, the Syrian Observatory for Human Right said.

The missiles, which were fired from Lebanese airspace, hit positions held by Iran-backed militias in the Masyaf district of Hama province, Observatory chief Rami Abdul Rahman said.

One also targeted a government-run research centre, where surface-to-surface missiles are developed and stored, the Britain-based watchdog said.

Iranian experts are believed to work in the research centre.

The Israeli military said it would not comment on reports in foreign media.

The research centre in Masyaf has been hit several times by Israeli strikes in recent years, the Observatory said.

According to the United States, sarin gas was being developed at the centre, a claim denied by Syrian authorities, who say the country has possessed no chemical weapons since it dismantled its arsenal under a 2013 agreement.

Syrian state news agency SANA said air defences intercepted missiles fired by Israel on Masyaf.

"Our air defences intercepted an Israeli attack on the Masyaf area," SANA reported.

It said air defences hit "most" missiles before they reached their target.

State television aired footage purporting to show air defences responding to the Israeli attack.

The Israeli activity in the skies was heard over parts of neighbouring Lebanon, where many took to social media to denounce the Christmas Day attack.

Israel has launched hundreds of strikes in Syria since the start of the civil war in 2011.

It has targeted government troops, allied Iranian forces and fighters from Lebanese militant group Hezbollah.

It rarely confirms details of its operations in Syria, but says Iran's presence in support of President Bashar al-Assad is a threat to which it will continue to respond.