BEIRUT — A strike Thursday by a suspected Iranian-made drone killed a U.S. contractor and wounded six other Americans in northeast Syria, and U.S. forces retaliated with airstrikes on sites in Syria used by groups affiliated with Iran's Revolutionary Guard, the Pentagon said. Activists said the U.S. bombing killed at least four people.

While it's not the first time the U.S. and Iran have traded strikes in Syria, the attack and the U.S. response threaten to upend recent efforts to deescalate tensions across the wider Middle East, whose rival powers have made steps toward détente in recent days after years of turmoil.

U.S. Defense Secretary Lloyd Austin said in a statement that the American intelligence community had determined the drone was of Iranian origin, but offered no other immediate evidence to support the claim. The drone hit a coalition base in the northeast Syrian city of Hasaka. The wounded included five American service members and a U.S. contractor.

Austin said the strikes were a response to the drone attack "as well as a series of recent attacks against coalition forces in Syria" by groups affiliated with the Revolutionary Guard.

Iran relies on a network of proxy forces through the Mideast to counter the U.S. and Israel, its arch regional enemy. The U.S. has had forces in northeast Syria since 2015, when they deployed as part of the fight against the Islamic State group, and maintains some 900 troops there, working with Kurdish-led forces that control around a third of Syria.

The U.S. airstrikes hit targets in three towns in eastern Syria, activists said. Overnight, videos on social media purported to show explosions in Deir el-Zour, a strategic province that borders Iraq and contains oil fields. Iranian-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 defense official, the U.S. counter strikes were conducted by F-15 fighter jets flying out of al-Udeid Air Base in Qatar. The official spoke on condition of anonymity to discuss military operations.

According to a U.S. official, the U.S. F-15s struck three locations, all in the vicinity of Deir el-Zour.

The activist group Deir Ezzor 24, which covers news in the province, said the American strikes killed four people and wounded a number of others, including Iraqis.

The Syrian Observatory for Human Rights, an opposition war monitor, put the death toll from U.S. strikes at 11 Iranian-backed fighters — including six at an arms depot in the Harabesh neighborhood in the city of Deir el-Zour and five others at military posts near the towns of Mayadeen and Boukamal.

Rami Abdurrahman who heads the Observatory said three rockets were fired earlier Friday at al-Omar oil field in Deir el-Zour that houses U.S. troops, an apparent retaliation to the American strikes.

The Associated Press could not immediately independently confirm the activist reports. Iran and Syria did not immediately acknowledge the strikes, nor did their officials at the United Nations in New York respond to requests for comment from the AP.

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.

The exchange of strikes came 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.

U.S. Army Gen. Michael "Erik" Kurilla, the head of the American military's Central Command, warned that its forces could carry out additional strikes if needed. "We are postured for scalable options in the face of any additional Iranian attacks," Kurilla said in a statement.

Addressing the U.S. House Armed Services Committee on Thursday, Kurilla warned lawmakers that 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.

"What Iran does to hide its hand is they use Iranian proxies," Kurilla said.

According to officials, Iran has launched 80 attacks against U.S. forces and locations in Iraq and Syria since January 2021. The vast majority of those have been in Syria.

Diplomacy to deescalate the exchange appeared to begin immediately. The foreign minister of Qatar spoke by phone with U.S. National Security Adviser Jake Sullivan as well as Iranian Foreign Minister Hossein Amirabdollahian, the Qatari state news agency report. Doha has been an interlocutor between Iran and the U.S. recently amid tensions over Tehran's nuclear program.

Austin said he authorized 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," Austin said. "No group will strike our troops with impunity."

The U.S. under Biden has struck Syria previously over tensions with Iran — in February and June of 2021, as well as August 2022.

Dareen Khalifa, a senior Syria analyst with the Brussels-based International Crisis Group, said that while Thursday's exchange of strikes comes at a sensitive political moment due to the "overall deterioration of U.S.-Iran relations and the stalling of the nuclear talks," she does not expect a significant escalation.

"These tit-for-tat strikes have been ongoing for a long time," Khalifa said, although she noted that they usually do not result in casualties.

While "the risk of an escalatory cycle is there," she said, "I think the Biden administration won't be eager to escalate in Syria now and will instead have a relatively measured response."

Since the U.S. drone strike that killed Revolutionary Guard Gen. Qassem Soleimani in 2020, Iran has sought "to make life difficult for U.S. 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," 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 Assad. The United Nations estimates over 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.