An Israeli minister has said that by striking key Iranian sites in Syria it sent a clear message that it will not tolerate an Iranian military foothold on its doorstep.

Minister of intelligence Israel Katz said it will take the Iranians time to "digest, understand and ask how Israel knew how to hit those sites".

After shooting down an Iranian drone that infiltrated its airspace, Israel launched a widespread offensive on Saturday in Syria.

The military said it hit four Iranian positions and eight Syrian sites, causing significant damage.

Israel has recently issued several stern warnings about the increased Iranian involvement along its borders with Syria and Lebanon.

Israel also said it destroyed the Syrian military's main command and control bunker in its most devastating assault there in decades.

The Syrian Observatory for Human Rights said the Israeli airstrikes killed at least six Syrian troops and allied militiamen.

It said on Sunday that they included Syrian troops as well as Syrian and non-Syrian allied troops.

The Observatory is based in Britain and monitors the war in Syria through activists on the ground.

The United States backed Israel and expressed deep concern over the latest escalation.

Heather Nauert, a State Department spokeswoman, said the US "strongly supports Israel's sovereign right to defend itself".

She 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."

"The US 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," she said.