JERUSALEM -- A Syria war monitor says an unprecedented wave of Israeli airstrikes in Syria killed at least six Syrian troops and allied militiamen.
The Syrian Observatory for Human Rights says the six were killed in Saturday's strikes, which the Israeli military said struck Iranian targets in several parts of Syria.
Sunday's report by the Observatory 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 neighboring Syria since fighting there began in 2011. Israel says the strikes were launched after it shot down an infiltrating Iranian drone.
The United States has backed Israel and expressed deep concern over the latest escalation. Heather Nauert, State Department spokeswoman, said late on Saturday that the U.S. "strongly supports Israel's sovereign right to defend itself."
Nauert said that "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 behavior that threatens peace and stability."
Israel's minister of intelligence says that by striking key Iranian sites in Syria, Israel has sent a clear message to Iran that it won't tolerate an Iranian military foothold on its doorstep. Israel Katz said on Sunday 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 says 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 says it destroyed the Syrian military's main command and control bunker in its most devastating assault there in decades.