Reports in Syrian and international media indicate that early Sunday, the Israeli Air Force (IAF) struck a military research facility on the outskirts of the Syrian capital of Damascus, shaking the city.

The airstrike was the second in three days, after the IAF reportedly attacked a weapons convoy inside Syria carrying Iranian-made long-range missiles to Hezbollah in Lebanon on Friday. Israel has yet to confirm the airstrikes officially.

However, former Israel Defence Forces intelligence chief Amoz Yadlin said the attacks sent a clear message to Iran that “at least some of the actors take red lines seriously,” according to the Times of Israel.

The comment referred to the “red line” set by U.S. President Barack Obama last year, which warned of consequences if Syria used chemical weapons in its civil war. Faced with evidence of such weapons, the president has dithered.

Despite his own indecision about U.S. responses to Syria, President Obama has appeared to give a green light to Israeli airstrikes against Syria, saying in an interview Saturday that the Israelis “have to guard against the transfer of advanced weaponry to terrorist organizations like Hezbollah.”

It is possible that U.S. intelligence assisted Israel, as it has done in the past, given that initial confirmation of the attacks came from U.S. officials.

Reports indicated that Sunday’s targets likely included the same Iranian-made missiles targeted in Friday’s airstrike, which could reach all of Israel’s major population centers from southern Lebanon–an Israeli “red line.”

Iran condemned the apparent Israeli airstrike on Syria but did not indicate whether or how it would respond, according to the Associated Press. Iran has been assisting the Syrian regime directly in its fight with rebel groups.

Though Israel has not officially acknowledged its role in the airstrikes, it moved rapidly to deploy Iron Dome anti-missile batteries to northern cities in preparation for possible retaliation from Hezbollah, which is backed by Iran and Syria.

Israeli Prime Minister Benjamin Netanyahu reportedly delayed a departure for China by to convene his security cabinet to review and approve measures to be taken in Israel’s self-defense, the Times of.