Four people were killed in an Israeli airstrike on Syria's Palmyra area in the province of Homs overnight into Thursday, according to the Syrian Observatory for Human Rights.
A Syrian soldier and three foreign citizens affiliated with pro-Iranian militias were killed in the attack, the watchdog said.
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Earlier, the Syrian Defense Ministry reported that one Syrian soldier was killed, and three others were wounded in the strike.
The attack targeted a communications tower and caused some material losses, the ministry added in a short statement on its Facebook page.
Iran-backed forces in Syria said on Thursday they would respond forcefully to Israel's strike.
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A statement by the so-called operations room of Assad's Iran-backed allies said the response to the strike would be "very cruel", adding casualties would have been much higher had its forces not been well spread across the desert area.
"As a result of this attack a number of martyrs and injured from our Mujahedeen brothers have fallen," the statement said without elaborating.
"We have taken a decision to respond to this attack in revenge for the martyrs and the blood of the injured and the response will be very cruel," said the statement published on pro-Iranian news outlets that Reuters verified for authenticity.
This is the second Israeli airstrike in Syria within a week, according to state media. On Saturday, a human rights watchdog reported that two Iran-backed militants were killed after Israel struck targets in Syria's military air base near Homs.
The airstrike also injured six soldiers and caused material damage, official Syrian media reported Friday.
"The Israeli enemy carried out an air aggression from the direction of the military area of Al-Tanf with bursts of missiles towards the T4 military airport in the central region," a source said, adding that the air defenses had brought down most of the missiles.
Israel has carried out hundreds of airstrikes inside Syria in the course of the country’s civil war, targeting what it says are suspected arms shipments believed to be bound for Lebanon’s Iran-backed Hezbollah militant group, which is fighting alongside Syrian government forces. It rarely acknowledges or discusses such operations.