A strike by a suspected Iranian-made drone killed a US contractor and wounded five American troops and another contractor in north-east Syria, the Pentagon said.
American forces said they retaliated soon after with “precision airstrikes” in Syria targeting facilities used by groups affiliated with Iran’s Revolutionary Guard, with activist groups saying they killed at least four people.
The attack and the US response threaten to upend recent efforts to deescalate tensions across the wider Middle East, whose rival powers have made steps toward detente in recent days after years of turmoil.
US defence secretary Lloyd Austin said in a statement the American intelligence community had determined the drone was of Iranian origin, but offered no other immediate evidence to support the claim.
“The airstrikes were conducted in response to today’s attack as well as a series of recent attacks against coalition forces in Syria” by groups affiliated with the Revolutionary Guard, Mr Austin said on Thursday evening.
Iran relies on a network of proxy forces throughout the Middle East to counter the US and Israel, its arch regional enemy.
The Pentagon said two of the wounded service members were treated on site, while three others and the injured contractor were taken to medical facilities in Iraq.
Overnight, videos on social media purported to show explosions in Syria’s Deir el-Zour, a strategic province that borders Iraq and contains oil fields.
Iran-backed militia groups and Syrian forces control the area, which also has seen suspected airstrikes by Israel in recent months allegedly targeting Iranian supply routes.
According to a defence official, the US counter strikes were conducted by F-15 fighter jets from the al-Udeid Air Base in Qatar.
The activist group Deir Ezzor 24 put the death toll from the American strikes at four people.
Deir Ezzor 24, which covers news in Deir el-Zour province, said the strikes hit the city of Deir el-Zour as well as militia posts near Mayadeen and Boukamal.
It said the strikes also hurt people, including Iraqis.
The Syrian Observatory for Human Rights, an opposition war monitor, said the American strikes killed six Iranian-backed fighters at an arms depot in the Harabesh neighbourhood in the city of Deir el-Zour.
The Observatory, which relies on a network of local contacts in Syria, said US bombing at a post near the town of Mayadeen killed two fighters.
A separate American strike hit a military post near the town of Boukamal along the border with Iraq, killing another three fighters, the Observatory said.
The Associated Press could not immediately independently confirm the activist reports.
Iran’s paramilitary Revolutionary Guard, which answers only to Supreme Leader Ayatollah Ali Khamenei, has been suspected of carrying out attacks with bomb-carrying drones across the wider Middle East.
In recent months, Russia has begun using Iranian drones in its attacks on sites across Ukraine as part of its war on Kyiv.
Iran has issued a series of conflicting denials about its drones being used in the war, though western nations and experts have tied components in the drones back to Tehran.
The exchange of strikes come as Saudi Arabia and Iran have been working toward reopening embassies in each other’s countries.
The kingdom also acknowledged efforts to reopen a Saudi embassy in Syria, whose embattled President Bashar Assad has been backed by Iran in his country’s long war.
US army General Michael “Erik” Kurilla, the head of the American military’s Central Command, said American forces could carry out additional strikes if needed.
“We are postured for scalable options in the face of any additional Iranian attacks,” he said.
Addressing the US House Armed Services Committee on Thursday, Gen Kurilla told politicians the “Iran of today is exponentially more militarily capable than it was even five years ago”.
He pointed to Iran’s arsenal of ballistic missiles and bomb-carrying drones.
Gen Kurilla also alleged that Iran had launched 78 attacks on US positions in Syria since January 2021.
“What Iran does to hide its hand is they use Iranian proxies,” he said.
Diplomacy to deescalate the crisis appeared to begin immediately around the strikes.
Qatar’s state-run news agency reported a call between its foreign minister and Jake Sullivan, the US national security adviser.
Doha has been an interlocutor between Iran and the US recently amid tensions over Tehran’s nuclear programme.
Qatar’s foreign minister also spoke around the same time with Iranian foreign minister Hossein Amirabdollahian.
Mr Austin said he authorised the retaliatory strikes at the direction of President Joe Biden.
“As President Biden has made clear, we will take all necessary measures to defend our people and will always respond at a time and place of our choosing,” he said.
“No group will strike our troops with impunity.”
The US under Mr Biden has struck Syria previously over tensions with Iran.
In February and June of 2021, as well as August 2022, Mr Biden launched attacks there.
US forces entered Syria in 2015, backing allied forces in their fight against the so-called Islamic State group.
The US still maintains the base near Hasakah in northeast Syria where Thursday’s drone strike happened.
There are roughly 900 US troops, and even more contractors, in Syria, including in the north and further south and east.
Since the US drone strike that killed Revolutionary Guard General Qassem Soleimani in 2020, Iran has sought “to make life difficult for US forces stationed east of the Euphrates”, said Hamidreza Azizi, an expert with the German Institute for International and Security Affairs.
“Iran increased its support for local proxies in Deir el-Zour while trying to ally with the tribal forces in the area,” Mr Azizi wrote in a recent analysis.
“Due to the geographical proximity, Iraqi groups also intensified their activities in the border strip with Syria and in the Deir el-Zour province.”
The strikes come during the Muslim holy month of Ramadan.
Syria’s war began with the 2011 Arab Spring protests that roiled the wider Middle East and toppled governments in Egypt, Libya, Tunisia and Yemen.
It later morphed into a regional proxy conflict that has seen Russia and Iran back Mr Assad.
The United Nations estimates more than 300,000 civilians have been killed in the war.
Those figures do not include soldiers and insurgents killed in the conflict; their numbers are believed to be in the tens of thousands.
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