DUBAI—U.S. airstrikes in Syria targeted an important way station in an Iranian network used to move weapons and fighters across the Middle East, from Tehran to Beirut.
Early Friday local time, American forces hit sites around Abu Kamal, just across the border from Iraq, in an area effectively controlled by Iranian-backed militias and used as a transit point for supplies.
The strikes hit supply trucks that were headed to an Iranian base nearby, according to Omar Abu Layla, who heads Deir Ezzour 24, a news website based in eastern Syria. Mr. Abu Layla described the base as “the logistical hub of the region.”
Kata’ib Hezbollah, an Iran-backed Shiite militant group, said one of its members was killed. The Syrian Observatory for Human Rights, a war monitoring group, said 22 militia members died, citing unidentified sources.
The American strike was a response to a rocket attack earlier this month on a U.S. air base in northern Iraq that killed a military contractor and injured others. A U.S. service member was among the wounded. A Shiite militia group claimed responsibility for the attack.
Iran didn’t comment on the airstrike in Syria and has denied any connection to the rocket attack on the U.S. air base. The Syrian government said the U.S. action showed “American disregard for the role of international law in solving the Syrian crisis.”
In Washington, the bombing rekindled a debate over the U.S. military’s authority to conduct such operations in foreign countries, with several Democratic lawmakers criticizing the strike.
“Offensive military action without congressional approval is not constitutional, absent extraordinary circumstances,” said Sen. Tim Kaine (D., Va.), who was joined in questioning the strikes by Sen. Chris Murphy (D., Conn.) and Rep. Ro Khanna (D., Calif.).
After a U.S. airstrike killed an Iranian commander, Qassem Soleimani, last year, Messrs. Kaine and Khanna co-sponsored resolutions to ban a strike on Iran without congressional approval, but the effort was vetoed by then-President Donald Trump.
Pentagon spokesman John Kirby said President Biden was authorized by Article 2 of the Constitution to defend U.S. personnel, as well as by Article 51 of the United Nations charter, which gives member states the right to self defense.
“The strikes were necessary to address the threat and [were] proportionate to the prior attacks,” Mr. Kirby said.
Mr. Kirby also addressed lawmakers who complained they hadn’t been adequately briefed on the strike beforehand, saying congressional briefings would be held in coming days.
Pro-Iran militant groups, with ties to Iran’s Islamic Revolutionary Guard Corps, have cemented their presence on both sides of the Syria-Iraq border since playing a key role in pushing Islamic State out of the area in 2017.
Iranian allies control a contiguous stretch of land in Iraq and Syria that permits the movement of military hardware and personnel from Iran all the way to Lebanon. Iran and associated militant groups also use airports in the region to transport weapons and people.
Israel is concerned that the free movement of weapons and personnel into Syria and Lebanon could threaten its security, and the Israeli air force has launched airstrikes in Syria in recent years in an effort to disrupt the supply chain.
Control of the al-Qaim border crossing between Iraq and Syria is also an important source of revenue for Iranian-backed groups. Such groups generate billions of dollars from payments collected at Iraq’s border crossings and airports, according to Iraqi officials.
As Mr. Biden moves to re-engage Tehran in negotiations over its nuclear program, he also must contend with an array of Iranian-backed armed groups that stretches across Iraq, Syria and Lebanon.
“They can mobilize fighters across a huge spectrum to start creating resistance groups and start targeting the U.S.,” said Renad Mansour, a senior research fellow at British think tank Chatham House and the author of a recent paper on the militant groups.
Friday’s airstrike appeared to be calibrated to impose a price for last week’s attack on U.S. forces without triggering an escalation or alienating Iraq’s political leadership.
The U.S. strike also tests its relationship with the other major foreign power in Syria, Russia. Russian military power helped turn the tide of the Syrian civil war in favor of President Bashar al-Assad.
Russian Foreign Minister Sergei Lavrov said the U.S. had warned the Russian military four or five minutes ahead of the airstrike, through a hotline set up between the two sides.
“If you’re speaking about deconfliction, as is customary between Russian and American soldiers, then this does nothing,” Russian news agencies quoted Mr. Lavrov as saying.
After a 2019 attack that killed a U.S. contractor in Iraq, the U.S. killed Mr. Soleimani, a top Iranian general, in a drone strike in January of last year.Iran retaliated with a missile attack on a base with U.S. forces in Iraq.
After those exchanges, Tehran moved Iranian and Russian-made short and medium-range missiles to parts of Syria along the Iraqi border, according to a person familiar with U.S. intelligence operations in the region.
The missiles, stored in an underground warehouse and other locations, were intended to deter the U.S. from attacking Iran’s strongholds along the Euphrates river in former Islamic State territories in the Syria-Iraq border region, the person said.
Iran’s mission to the United Nations didn’t immediately respond to a request for comment.
In Syria, Iran’s Revolutionary Guard and its allied militias played a central role in helping Mr. Assad recapture much of the country from rebels, operations that were orchestrated by Mr. Soleimani.
Iran has used private Syrian security companies to secure highways. The companies have helped create supply lines that facilitate the movement of people and military hardware between Iran and Syria.
“If you cross one line, from opposition areas to regime-controlled areas, you are basically in Tehran,” said Manhal Bareesh, a Syrian researcher who wrote a paper on Syrian security companies for the European University Institute.
—Sune Engel Rasmussen in London, Nazih Osseiran in Beirut, Ghassan Adnan in Baghdad, and Gabriel T. Rubin and Nancy A. Youssef in Washington contributed to this article.
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