BEIRUT (AP) — Israeli warplanes carried out intense airstrikes in eastern Syria early Wednesday, apparently targeting positions and arms depots of Iran-backed forces as the region is on high alert. At least 57 fighters were killed and dozens were wounded, according to a Syrian opposition war monitoring group.

A senior U.S. intelligence official with knowledge of the attack told The Associated Press that the airstrikes were carried out with intelligence provided by the United States — a rare incidence of publicized cooperation between the two countries over choosing targets in Syria. The official said the strikes targeted a series of warehouses in Syria that were being used in a pipeline to store and stage Iranian weapons.

The U.S. official, who requested anonymity to speak about the matter, said U.S. Secretary of State Mike Pompeo discussed Tuesday’s airstrike with Yossi Cohen, chief of Israel’s spy agency Mossad, at a public meeting in the popular Washington restaurant Café Milano on Monday.

The warehouses also served as a pipeline for components that support Iran’s nuclear program, the official said. There was no immediate comment from Iran.

Syria’s state news agency SANA said the strikes hit areas in and near the towns of Deir el-Zour, Mayadeen and Boukamal along the border with Iraq. An unnamed military official was quoted as saying Syrian air defenses responded to the incoming missiles. It gave no further details.

A Syrian opposition war monitoring group reported at least 18 strikes in Deir el-Zour and along the border with Iraq, saying several arms depots were hit. The Britain-based Syrian Observatory for Human Rights said 57 people were killed, including 14 Syrian troops, and the rest were Iran-backed fighters including 16 Iraqis and 11 Afghans. Dozens were wounded.

The death toll could not be independently verified. If confirmed, it would make it one of the deadliest Israeli strikes in Syria over the past 10 years.

“They burned Iranian positions in Deir el-Zour,” said Omar Abu Laila, a Europe-based activist from Syria’s eastern Deir el-Zour province who runs an activist collective that reports on news in the border area. He recorded at least 16 targeted buildings, warehouses or bases for Iranian, Lebanese and Iraqi militias in the towns of Boukamal, Mayadeen and Deir el-Zour.

Israel has launched hundreds of strikes against Iran-linked military targets in Syria over the years but rarely acknowledges or discusses such operations.

The Observatory said it recorded 39 Israeli strikes inside Syria in 2020 that hit 135 targets, including military posts, warehouses or vehicles. In those attacks, at least 217 people were killed, mostly Iranian-allied militiamen, according to the Observatory.

The strikes come at a time of heightened tension in the region in the final days of President Donald Trump’s administration.

The Israeli military has been on a high level of defensive alert, and according to Amos Harel, a prominent Israeli military affairs correspondent, an aerial defense battery of Patriot missiles was deployed in Israel’s southern seaport of Eilat. Writing in the newspaper Haaretz, he said an exceptionally large presence of fighter planes has been in the skies over the country for a considerable portion of the day.

Meanwhile, Israeli jets have been violating Lebanese airspace and crisscrossing skies over Beirut in daily, low-altitude flights that have added to jitters in the Lebanese capital.

Many fear retaliation for the U.S. killing of Iran's Revolutionary Guard commander Qassem Soleimani last year in Baghdad may come before Trump leaves office, or that conflict could erupt to scuttle efforts by Joe Biden’s incoming administration to negotiate with Iran.

Israel views Iranian entrenchment on its northern frontier as a red line, and has repeatedly struck Iran-linked facilities and weapons convoys destined for Lebanon's militant Hezbollah group.

The strikes also come amid intensifying low-altitude Israeli warplane missions in Lebanese skies that have caused jitters among residents and prompted Lebanon to file an urgent complaint to the U.N. about the violations of its airspace. Israeli officials have said the overflights are necessary because Hezbollah is violating the 2006 U.N. resolution that bars it from building up its military capabilities and operating near the Israeli border.

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Associated Press writer James LaPorta in Delray Beach, Florida, contributed to this report.