Israeli missiles tore across Lebanon early on Friday morning, hitting targets in Syria and killing six Iran-backed fighters, a war monitor has reported.
The dead were all foreign paramilitaries fighting alongside President Bashar Al Assad's forces, the UK based Syrian Observatory for Human Right said.
"At 12.40 this morning, the Israeli enemy launched an aggression by directing a barrage of missiles from the north of the Lebanese city of Tripoli towards the Masyaf area in the western Hama countryside," the Syrian military said in a statement reported by state TV.
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.
Videos purporting to show the missiles crossing Lebanon were shared online.
Israeli jets regularly violate Lebanese airspace and have often struck inside Syria from Lebanese territory.
But the Christmas Eve flights were louder than usual, frightening residents of Beirut who have endured multiple crises in the past year, including the catastrophic August 4 explosion at the city's port that killed over 200 people and destroyed parts of the capital.
That explosion resulted from the detonation of a stockpile of ammonium nitrate that was improperly stored at the port.
One of Friday's missiles also targeted a government-run research centre, where surface-to-surface missiles are developed and stored, the 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.
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 Mr Al Assad is a threat to which it will continue to respond.