Twenty-one people were killed in Israeli retaliatory airstrikes in Syria early on Monday, 12 of them members of the Iran’s Revolutionary Guard Corps, a Britain-based Syrian war monitor said Tuesday.

On Sunday, Israel reportedly conducted a rare daylight missile attack on Iranian targets in Syria. In response, Iran fired a surface-to-surface missile at the northern Golan Heights, which was intercepted by the Iron Dome missile defense system over the Hermon ski resort, according to the Israel Defense Forces.

Hours later, in the predawn hours of Monday morning, the Israeli Air Force launched retaliatory strikes on Iranian targets near Damascus and on the Syrian air defense batteries that fired upon the attacking Israeli fighter jets, the army said.

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The UK-based Syrian Observatory for Human Rights initially reported the death toll from the Israeli strikes to be 11. But on Tuesday, the war monitor said the number had risen to 21, making it one of the deadliest attacks by Israel in Syria.

According to SOHR, 12 of those killed were members of Iran’s Islamic Revolutionary Guard Corps; six were Syrian military fighters; and the other three were other non-Syrian nationals.

In July 2018, 22 people, nine of them Iranians, were said to have been killed in an airstrike attributed to Israel on an Iranian-controlled base in northern Syria.

In May 2018, in an air battle sparked by Iran launching dozens of rockets at the Golan, at least 23 fighters were killed in Syria. Eighteen of them were said to be foreigners, though it was not immediately clear how many were Iranians and how many were non-Syrian nationals from elsewhere in the Muslim world fighting in Shiite militias.

Escalating attacks

The IDF said Monday that Iranian troops in Syria launched their missile at the Golan in a “premeditated” attack aimed at deterring Israel from conducting airstrikes against the Islamic Republic’s troops and proxies in Syria.

Israeli troops on Monday were put on high alert in the north.

Military spokesperson Jonathan Conricus said the three response sorties destroyed a number of Iranian intelligence sites, training bases and weapons caches connected to the Quds Force, the expeditionary arm of Iran’s Islamic Revolutionary Guard Corps.

According to Conricus, one of the targets of the raids was “the main storage hub for Quds Force.”

On Monday morning, the IDF also released video footage of its airstrikes on Syrian air defenses, including on social media.

תיעוד מתוך תקיפת חלק מסוללות ההגנה האווירית הסוריות לאחר שביצעו ירי הלילה: pic.twitter.com/E6bhJ9bDw7 — צבא ההגנה לישראל (@idfonline) January 21, 2019

According to Conricus, the Iranian retaliatory strike aimed at the northern Golan was “not a spur-of-the-moment” response, but had been planned months in advance, based on intelligence collected by the IDF.

“We understand that the Iranians are trying to change the context and deter us from our policy and our strategy of fighting Iranian troops in Syria,” Conricus said. “They thought they could change the rules of engagement. Our response was a rather clear one, with a message to Iran and Syria that our policies have not changed.”

He acknowledged that while the military believed it was planned in advance, the trigger for Sunday’s attack was likely the airstrikes reported moments before.

The spokesman said Iran was directly responsible for the launch, and disputed reports that the projectile had been fired by pro-Iranian militias or by the Syrian regime.

Conricus said the location from which the missile was fired was “an area that we have been promised that the Iranians would not be in.”

That assurance appeared to have been made by Russia — Syrian dictator Bashar Assad’s prime ally in the civil war — but Conricus said he “won’t go into who made the promise.”

Israel has reached a number of understandings with Russia about the permitted location of Iranian troops in Syria, mostly about their deployment along the Golan border with Syria.

The IDF spokesperson said the military ultimately holds Syria responsible for the attack and warned that the country would “pay the price” for allowing Iran to establish a permanent military presence in its territory. Iran officially denies having troops in Syria beyond a small number of advisers — a claim that is widely disregarded among Western intelligence officials.