Israeli strikes on several areas of Syria overnight killed at least 23 fighters, including five Syrian regime troops and 18 other allied forces, a monitor said Thursday.

The Britain-based Syrian Observatory for Human Rights said the regime troops killed in the strikes included an officer, adding that the other casualties included Syrians and foreigners.

It didn’t say whether any Iranians were among the foreigner casualties.

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Syria’s military denied the Observatory’s report, saying the Israeli airstrikes killed three people and wounded two others, destroyed a radar station and an ammunition warehouse, and damaged a number of air defense units.

Some 20 rockets were fired at northern Israeli military bases by Iranian forces from southern Syria just after midnight, Israel said, prompting extensive retaliatory raids. The Israel Defense Forces said that it suffered no casualties, either on the ground or in the air, and that no rockets fired from Syria made impact in Israeli territory.

Russia’s defense ministry said Israel’s strikes on Syria saw 28 planes take part in raids with a total of around 70 missiles fired.

“28 Israeli F-15 and F-16 aircraft were used in the attack, which released around 60 air-to-ground missiles over various parts of Syria. Israel also fired more than 10 tactical ground-to-ground missiles,” the ministry said in a statement quoted by the Interfax news agency.

The IDF said it had hit dozens of Iranian military targets in Syria overnight.

Defense Minister Avigdor Liberman said that the IDF had destroyed “nearly all” of Iran’s military infrastructure sites in Syria.

Four of the 20 projectiles launched by Iranian troops at Israel were intercepted by the Iron Dome missile defense system and the rest fell in Syria, IDF spokesperson Jonathan Conricus said. The rockets included both Grad and Fajr-5 models, according to the military.

The IDF said the initial missile barrage was launched by members of the Iranian Revolutionary Guard Corps’ al-Quds Force. It appeared to be the first time Israel attributed an attack directly to Iran, which generally operates through proxies.

In all, the army said it carried out approximately 50 retaliatory raids against IRGC targets, including intelligence centers, weapons depots, storage facilities, observation posts, and logistics centers in Syria, as well as the rocket launcher that carried out the initial attack.

The overnight exchange was the largest-ever direct clash between the Iranian and Israeli militaries, and appeared to be the largest exchange involving Israel in Syria since the 1973 Yom Kippur War.

The military said it also targeted a number of Syrian air defense systems — SA-5, SA-2, SA-22 and SA-17 batteries — that had fired at Israeli planes, despite the military’s Arabic-language spokesperson explicitly warning earlier that “any Syrian involvement will be met with the utmost severity.”

In the days and weeks before the Iranian barrage, defense officials repeatedly warned that Israel would respond aggressively to any attack from Syrian territory.

Tehran has repeatedly vowed revenge after the T-4 army base in Syria was struck in an air raid — widely attributed to Israel — on April 9, killing at least seven members of the IRGC, including a senior officer responsible for the group’s drone program.