Israeli Prime Minister Benjamin Netanyahu says his country's military has dealt "severe blows" to Iran and Syria with a series of airstrikes.

He told his cabinet on Sunday that Israel "clarified unequivocally" that it will "continue to strike any attempt to strike us."

"That has been our policy and that will continue to be our policy," he said, a day after the airstrikes.

Saturday's strikes followed Israel's interception of an Iranian drone that had infiltrated its airspace.

Also, an Israeli F-16 warplane was downed upon its return from Syria after coming under what Israel described as "massive anti-aircraft fire" from Syrian forces.

MIDEAST-CRISIS/SYRIA-ISRAEL

Israeli security forces examine the remains of an F-16 Israeli war plane near the village of Harduf, Israel February 10, 2018. REUTERS/ Ronen Zvulun - RC1BFFED0EC0 (Ronen Zvulun/Reuters)

A Syria war monitor says the Israeli airstrikes in Syria killed at least six Syrian troops and allied militiamen.

The Israeli military said it hit four Iranian positions and eight Syrian sites, causing significant damage. Among the targets destroyed was the Syrian military's main command and control bunker.

Sunday's report by the Syrian Observatory for Human Rights says they included Syrian troops as well as Syrian and non-Syrian allied troops. The Britain-based Observatory monitors the war in Syria through a network of activists on the ground.

The Israeli strikes marked Israel's most serious engagement in neighbouring Syria since fighting there began in 2011. 

The United States has backed Israel and expressed deep concern over the latest escalation. State Department spokesperson Heather Nauert said late Saturday that the U.S. "strongly supports Israel's sovereign right to defend itself."

Nauert said "Iran's calculated escalation of threat and its ambition to project its power and dominance, places all the people of the region — from Yemen to Lebanon — at risk."

She said the "U.S. continues to push back on the totality of Iran's malign activities in the region and calls for an end to Iranian behaviour that threatens peace and stability."