Five officers of Iran’s Revolutionary Guards, including the commander, were killed in an Israeli airstrike on the outskirts of Damascus overnight Thursday into Friday, a non-governmental organization said.

“Five officers of the Iranian Revolutionary Guards, including a senior officer, were killed when Israeli strikes hit a building along a southern highway” shortly after they arrived there, Rami Abdel Rahman, the head of the Syrian Observatory for Human Rights, told the German news agency.

It was the second Israeli raid on the Syrian capital before even 24 hours had passed. Syrian state media reported two wounded Syrian soldiers in the previous raid, overnight Wednesday into Thursday.

Iran’s semi-official Tasnim news agency reported Friday a statement from the Revolutionary Guards confirmed that an officer serving as a military adviser in Syria was killed in an Israeli raid on the outskirts of Damascus. The Revolutionary Guards declared that they would retaliate for what they described as a “crime of the Zionist regime”.

The Revolutionary Guards named the officer as Milad Haidari.

Iran has been among the most important allies of Syrian President Bashar al-Assad’s regime since war broke out in Syria in 2011 and has deployed trainers and “advisors” of its armed forces to the country.

Iranian Foreign Ministry spokesman Nasser Kanani also condemned the Israeli attacks on Thursday and Friday.

Since the war in Syria broke out in 2011, Israel has launched hundreds of strikes on its territory, the vast majority of them from the air, against positions of the Syrian armed forces and their allies, Iranian forces and pro-Iranian armed groups, especially Lebanon’s Hezbollah. He rarely confirms his operations in this undeclared war, however he often declares that he will not allow Iran, his sworn enemy, to build a bridgehead or extend its influence in his neighboring country.

The highly complex war in Syria entered its 13th year earlier this month; it has claimed the lives of more than half a million people, turned millions more into internally displaced persons and refugees, and caused a massive economic and humanitarian crisis.