Experts said Thursday that a series of suspicious flights possibly carrying arms or fighters from Iran landed in Damascus shortly before a reported Israeli missile strike on a military site near the Syrian capital’s airport.

Syria’s military said Israel struck a military installation southwest of Damascus International Airport before dawn Thursday, setting off a series of explosions.

An unnamed regional intelligence source, quoted by Reuters, said the strike was carried out by Israel and targeted an Iran-supplied Hezbollah arms depot.

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Just before the apparent Israeli missile strike, at least three cargo jets from Iran probably landed at the Damascus airport, said Ian Petchenik, a spokesman for the flight-tracking website FlightRadar24. They include an Il-76 flown by the Iranian cargo company Pouya Air that “was last tracked over Iraq headed towards Damascus,” he said.

It’s unknown what they were carrying. Passenger flights and civilian cargo jets continue flying into Damascus, although there’s suspicion that some commercial flights serve as cover for weapons transfers from Iran.

The Washington-based Foundation for Defense of Democracies, a right-leaning think tank that has criticized the nuclear deal Iran struck with world powers, has said Pouya Air is the latest name for a long-sanctioned airline.

It also has accused Pouya Air of funneling arms from Iran into Yemen’s capital of Sanaa to supply Shiite rebels there.

Emanuele Ottolenghi, a senior fellow at the foundation, said he tracked a fourth cargo flight from Iran to Syria on Wednesday night, an Airbus A300 operated by Mahan Air, which is suspected of ties to Iran’s paramilitary Revolutionary Guard.

He also called one of the cargo flights, a Qeshm Fars Air Boeing 747, especially suspicious because the airline stopped operating in 2013, only to resume flights to Damascus three weeks ago.

“We don’t know for sure, but let’s say that we can fairly safely assume that the weaponry and fighters reach Damascus through these daily flights,” Ottolenghi told The Associated Press.

In line with its usual practice, Israel’s military declined to comment on the latest blast, which created a huge explosion.

Earlier Thursday, Intelligence Minister Yisrael Katz appeared to confirm that Israel was behind the overnight strike.

Katz, who is also transportation minister, told Army Radio in an interview that “the incident is completely compatible with our policy of preventing weapons transfer to Hezbollah,” the Lebanon-based terror group supported by the Syrian regime and Iran.

“Every time we receive intelligence information on plans to transfer advanced weaponry to Hezbollah, we will act,” the minister added. “We must prevent Iran from establishing a military presence in Syria.”

The explosions near Damascus reverberated across the capital, the seat of Assad’s power.

Syria’s state-run SANA news agency said Israel had fired several missiles from inside the Golan Heights, 60 kilometers (37 miles) south of Damascus, striking a military installation southwest of the airport that serves both military and civilian flights. It reported damage but no casualties.

“The buildings shook from the force of the blast,” said a media activist who goes by Salam al-Ghoutawi of the Ghouta Media Center in the opposition-held northeastern suburbs, about 15 kilometers (9 miles) from the airport. He said he heard the roar of jets in the distance.

Explosions were silhouetted against the night sky in a video published by the center. Debris was seen flying out as the explosions illuminated a sizeable cloud nearby.

Hezbollah’s al-Manar media station reported a blast at fuel tanks and a warehouse next to the airport, which is 25 kilometers (16 miles) east of central Damascus.

The Syrian military said in a statement the attack sought to “raise the morale of terrorist groups” the government maintains are fighting Assad’s forces. It made no mention of whether it would respond.

Israel is widely believed to have carried out airstrikes in recent years on advanced weapons systems in Syria — including Russian-made anti-aircraft missiles and Iranian-made missiles — as well as on Hezbollah positions. It rarely comments on such operations.

Later Thursday, Israel said it fired a Patriot missile to down a Syrian drone over the Golan Heights.

According to the IDF, the Syrian UAV was under “full [Israeli Air Force] surveillance” while it was in Israeli airspace.