WASHINGTON — The conflict in northeast Syria escalated on Friday as Iran-backed militias launched a volley of rocket and drone attacks against coalition bases after American reprisals for a drone attack that killed a U.S. contractor and injured six other Americans.
President Biden, speaking at a news conference in Canada, sought to tamp down fears that tit-for-tat strikes between the United States and militant groups could spiral out of control, while at the same time warning Tehran to rein in its proxies.
“Make no mistake, the United States does not, does not, I emphasize, seek conflict with Iran,” Mr. Biden said in Ottawa, where he was making a state visit. “But be prepared for us to act forcefully to protect our people. That’s exactly what happened last night.”
The fighting, among the most serious in the area since 2019, threatens to upend recent efforts to de-escalate tensions across the wider Middle East, whose rival powers, including Iran and Saudi Arabia, have made steps toward rapprochement in recent days after years of turmoil.
The initial attack on Thursday came as American forces in northeast Syria were on high alert following 78 attacks by Iran-backed militias since January 2021. But a self-destructing drone, which American officials said was of “Iranian origin,” managed to hit a coalition base anyway, killing the U.S. contractor and wounding six other Americans.
Two U.S. officials said the main air defense system at the base was “not fully operational” at the time, raising questions about whether the attackers had detected that vulnerability and exploited it, or just happened to send the drone at that time, according to people who spoke on condition of anonymity to discuss the investigation.
Brig. Gen. Patrick Ryder, the Pentagon spokesman, said that the air defense’s radar was working but he declined to discuss any other details of the system, citing operational security and an investigation by the military’s Central Command.
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It was unclear why the system was not fully functional and what difference that had made in defending the base. The Avenger missile defense system at the base, called RLZ, may have been experiencing a maintenance problem, one of the U.S. officials said.
The base near Hasaka has other defenses, but even all those systems combined are not foolproof, officials said. Pentagon and other American officials said they were reluctant to discuss any possible weaknesses or failings in the layered defense network to avoid giving adversaries in the region any advantage.
“We take a variety of measures to safeguard our people, but again, it’s an inherently dangerous place,” General Ryder said.
After U.S. intelligence analysts concluded that the drone was of Iranian origin — a claim the Pentagon made without any supporting evidence — the United States retaliated by launching airstrikes against militant sites linked to Iran’s Islamic Revolutionary Guards Corps, General Ryder said.
Two Air Force F-15E fighter jets hit a munition warehouse and a control building nearby, and an intelligence-collection site in eastern Syria, two senior U.S. military officials said.
The U.S. airstrikes killed eight pro-Iran fighters, according to the Syrian Observatory for Human Rights, a group in Britain that tracks the conflict through contacts in Syria. General Ryder said the military was still investigating reports of casualties on the ground.
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“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,” Defense Secretary Lloyd J. Austin III said Thursday. “No group will strike our troops with impunity.”
Iran-backed militias responded by firing rocket and drone attacks on coalition targets, including at a second U.S. base in the area, called Green Village. Another American was injured in one of the attacks, U.S. officials said, raising fears that the violence could escalate into a wider war.
Asked whether the United States holds Tehran responsible for the death of an American citizen, General Ryder said, “Iran certainly backs these groups, and by default, therefore has a responsibility to ensure that they’re not contributing to insecurity, instability. But clearly, they continue to do that.”
Charles Lister, the director of the Syria and Countering Terrorism and Extremism programs at the Middle East Institute in Washington, said that the targeted American retaliatory strikes were important but unlikely enough to deter Iran’s behavior or the actions of its proxies.
“Until Iran senses that such attacks reap unsustainable consequences, they will continue,” Mr. Lister said in an email. “That presents President Biden with a political question: Is he willing to push back more determinedly to assert a deterrent effect, or are we willing to allow these repeated attacks on U.S. forces?”
The Islamic Revolutionary Guards Corps is a powerful branch of Iran’s armed forces that operates in parallel with the military. It is charged with securing Iran’s borders, and its overseas arm, the Quds Force, carries out operations across the Middle East and beyond, and trains and arms Shiite proxy militias that operate in a number of countries. The U.S. has designated it a terrorist group.
Iran has built increasingly sophisticated weapons-capable drones in recent years. It has both sold them commercially to other nations, including to Russia for use in the war in Ukraine, and stepped up their transfer to proxy groups.
The drones are part of a rapidly evolving threat from Iranian proxies in Syria, with militia forces specialized in operating more sophisticated weaponry hitting some of the most sensitive American targets in attacks that evaded U.S. defenses.
Two of the wounded U.S. service members were treated on site, while the three other service members and the contractor were medically evacuated to coalition medical facilities in Iraq. The Pentagon did not identify the contractor who was killed, pending notification of family, a senior military official said.
America still has more than 900 troops, and hundreds more contractors, in Syria, working with Kurdish fighters to make sure there is no resurgence of the Islamic State, which was ostensibly defeated as a self-declared caliphate in 2019, after five years of wreaking havoc across Iraq and Syria.
Iranian-backed militias have launched dozens of attacks at or near bases where U.S. troops are in the past year alone. Since January 2021, Gen. Michael E. Kurilla, the head of the Central Command, said Iran-backed groups had carried out 78 attacks against Americans before Thursday’s strike.
“Iran’s vast and deeply resourced proxy forces spread instability throughout the region and threaten our regional partners,” General Kurilla, who earlier this month visited the same base in northeast Syria that was attacked, told the House Armed Services Committee on Thursday.
U.S. and partner forces with a coalition that includes the Kurdish Syrian Democratic Forces have been working together to keep pressure on Islamic State militants and to ensure that detained fighters do not end up back on the battlefield.
The Kurdish Syrian forces conduct targeted raids against Islamic State members. They also guard more than 10,000 imprisoned Islamic State fighters, while the Pentagon and American troops provide air support, intelligence and reconnaissance.
With the Biden administration focused on Russia’s invasion of Ukraine and a potential future conflict with China, the counter-Islamic State military mission in Syria has become something of a back-burner issue.
The mission has received greater attention only when Iranian-backed militias or Islamic State militants attack the American troops who rotate in and out, for nine months at a time, across a handful of bases in northeast Syria, which Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, visited this month.
The United States has repeatedly carried out airstrikes in response. In June 2021, it struck facilities used by two Iranian-backed militias in Iraq and Syria that the Pentagon said had conducted drone strikes against American personnel in Iraq. In December 2019, the U.S. military struck five targets in Iraq and Syria controlled by an Iranian-backed paramilitary group in retaliation for a rocket attack that killed an American contractor.
John Yoon contributed reporting from Seoul, Hwaida Saad from Syria, and Michael D. Shear from Ottawa.
