BEIRUT (AP) — Iran-backed fighters were on alert in eastern Syria on Saturday, a day after the United States (US) forces launched retaliatory airstrikes on sites in the war-torn country, opposition activists said. The airstrikes came after a suspected Iran-made drone killed a US contractor and wounded six other Americans on Thursday.

The situation was calm following a day in which rockets were fired at bases housing US troops in eastern Syria. The rockets came after US airstrikes on three different areas in Syria’s eastern province of Deir el-Zour, which borders Iraq, opposition activists said.

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

In the first Syrian comment since the attacks occurred, the foreign ministry condemned the US strikes saying they are “a flagrant violation of Syria’s sovereignty.” The ministry added in a statement that Damascus aims to end “American occupation” and spread government control over all parts of the country.

“The calm continues as Iran-backed militiamen are on alert out of concern of possible new airstrikes,” said Rami Abdurrahman, who heads the Syrian Observatory for Human Rights, an opposition war monitor.

Omar Abu Layla, an activist from Deir el-Zour who is currently in the US and runs a group that monitors developments in east Syria, said Iran-backed fighters have evacuated some of their posts and moved into trenches in case of any more American strikes. “They are in a state of alert,” he said.

President Joe Biden said Friday that the US would respond “forcefully” to protect its personnel after US forces retaliated with airstrikes on sites in Syria used by groups affiliated with Iran’s Revolutionary Guard. The strikes followed an attack Thursday by a suspected Iran-made drone that killed a US contractor and wounded five American servicemembers and a US contractor.

“The United States does not, does not seek conflict with Iran,” Biden said in Ottawa, Canada, where he was on a state visit. But he said Iran and its proxies should be prepared for the U.S. “to act forcefully to protect our people. That’s exactly what happened last night.” Activists said the US bombing killed at least four people.

In Iran, domestic media outlets quoted a spokesman for the nation’s Supreme National Security Council, Keivan Khosravi, as saying that Tehran would immediately respond to any US attack on Iranian bases in Syria.

“Any excuses-seeking attitude for attack on bases that are established at the request of the Syrian government, will immediately face an answer,” Khosravi was quoted as saying. Khosravi rejected U.S charges that Iran is behind attacks on American bases in Syria, suggesting they are attacks against “illegal occupation of part of Syria.”

A statement issued late Friday by the Iranian Consultative Center in Syria warned the US not to carry out further strikes in Syria. Otherwise, “we will have to retaliate.” It warned that “it will not be a simple revenge.”

The centre, which speaks on behalf of Tehran in Syria, said the US airstrikes targeted places used to store food products and other service centres in Deir el-Zour. It said the strike killed seven people and wounded seven others without giving the nationalities of the dead. An official with an Iran-backed group in Iraq said the strikes killed seven Iranians.

The Observatory raised the death toll from the US strikes to 19, saying they were killed in three locations, including an arms depot in the Harabesh neighbourhood in the city of Deir el-Zour, and two military posts near the towns of Mayadeen and Boukamal.

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 US officials, two simultaneous attacks were launched at US forces in Syria late Friday. Officials said that based on preliminary information, there was a rocket attack on the Conoco plant, where U.S. troops are stationed, and one US service member was wounded but is in stable condition. At about the same time, several drones were launched at Green Village, in Deir el-Zour province where US troops are also based. One official said all but one of the drones were shot down, and there were no US injuries there. The officials spoke on condition of anonymity to discuss military operations.

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.

US Defense Secretary Lloyd Austin said the American intelligence community had determined the drone in Thursday’s attack 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 Hasakeh.

Iran relies on a network of proxy forces throughout the Mideast to counter the US and Israel, its arch regional enemy. The US 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 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.

The State Department said in a statement Saturday that they have been in contact with Saudi officials about their approach to the region, including with Syria adding that Washington’s stance against normalisation with Assad’s government “remains unchanged, and we have been clear about this.”

The State Department said the US’s consistent message to regional partners who are engaging with the Syrian government has been that “credible steps to improve the situation for the Syrian people should be front and centre in any engagement.”

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

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

Syria’s conflict that began in 2011 has left nearly half a million people dead.