The U.S. military on Sunday shot down a Syrian Air Force fighter jet that bombed local forces aligned with the Americans in the fight against ISIS militants, an action that appeared to mark a new escalation of the conflict.

The U.S.-led coalition headquarters in Iraq said in a written statement that a U.S. F-18 Super Hornet shot down a Syrian government SU-22 after it dropped bombs near the U.S. partner forces, known as the Syrian Democratic Forces. The shootdown was near the Syrian town of Tabqa.

The U.S. military statement said it acted in “collective self defence” of its partner forces and that the U.S. did not seek a fight with the Syrian government or its Russian supporters.

According to a statement from the Pentagon, pro-Syrian regime forces attacked the U.S.-backed Syrian Democratic Forces-held town of Ja’Din, south of Tabqah in northern Syria, wounding a number of SDF fighters and driving the SDF from the town.

Coalition aircraft conducted a show of force and stopped the initial pro-regime advance toward the town, the Pentagon said. Following the pro-Syrian forces attack, the coalition called its Russian counterparts “to de-escalate the situation and stop the firing,” according to the statement.

A few hours later, the Syrian SU-22 dropped bombs near SDF fighters and, “in collective self-defence of coalition-partnered forces,” was immediately shot down by a U.S. F/A-18E Super Hornet, the Pentagon said.

“The coalition’s mission is to defeat ISIS in Iraq and Syria,” the Pentagon said. “The coalition does not seek to fight Syrian regime, Russian or pro-regime forces partnered with them, but will not hesitate to defend coalition or partner forces from any threat.”

U.S. forces tangled earlier this month with Syria-allied aircraft in the region. On June 8, U.S. officials reported that a drone likely connected to Iranian-supported Hezbollah forces fired on U.S.-backed troops and was shot down by an American fighter jet. The incident took place in southern Syria near a base where the U.S.-led coalition was training Syrian rebels fighting ISIS.

An Army spokesperson at the Pentagon said at the time that the drone carried more weapons and was considered a direct threat, prompting the shootdown.

Moscow condemned the downing of the Syrian jet and Russia’s Defence Ministry said Monday that warplanes from the U.S.-led coalition operating over Syrian government-controlled areas west of the Euphrates River will be tracked as potential targets,

Areas of northern Syria west of the Euphrates were controlled by ISIS before Syrian government forces captured most of them in recent months. The Russians appear to want to avoid further U.S. targeting of Syrian warplanes or ground troops that have come under U.S. attack in eastern Syria recently.

Moscow also called on the U.S. military to provide a full accounting of why it decided to shoot down the Syrian Su-22.

Russia, a key ally of Syrian President Bashar al-Assad, has been providing an air cover to the government’s offensive since 2015.

Iran responds to Tehran attack

But in April, Russia briefly suspended a hotline intended to prevent midair incidents with the U.S. over Syria after the American military fired 59 missiles at a Syrian airbase following a chemical weapons attack that Washington blamed on the Assad government.

In comments to Russian news agencies, Russian Deputy Foreign Minister Sergei Ryabkov compared the downing to “helping the terrorists that the U.S. is fighting against.”

“What is this, if not an act of aggression?” he asked.

Viktor Ozerov, chairman of the defence and security committee at the upper chamber of Russian parliament, described the Defence Ministry’s statement as a warning.

“I’m sure that because of this neither the U.S. nor anyone else will take any actions to threaten our aircraft,” he told the state-owned RIA Novosti news agency. “That’s why there’s no threat of direct confrontation between Russia and American aircraft.”

Ozerov insisted that Russia will be tracking the coalition’s jets, not shooting them down, but he added that “a threat for those jets may appear only if they take action that pose a threat to Russian aircraft.”

Meanwhile, Iran launched a ballistic missile strike in Syria’s eastern city of Deir el-Zour on Sunday night, which appeared to be Iran’s first missile attack abroad in over 15 years and its first in the Syrian conflict, in which it has provided crucial support to embattled President Bashar Assad.

Iran says its ballistic missile strike targeting the Islamic State group in Syria was not only a response to deadly attacks in Tehran recently, but a powerful message to archrival Saudi Arabia and the United States, one that could add to already soaring regional tensions.

The missiles flew over Iraq before striking what Iran’s Revolutionary Guard called an Islamic State command centre and suicide car bomb operation in Deir el-Zour, over 600 kilometres away. The extremists have been trying to fortify their positions in the Syrian city in the face of a U.S.-led coalition onslaught on Raqqa, the group’s de facto capital.

Syrian opposition activist Omar Abu Laila, who is based in Germany but closely follows events in his native Deir el-Zour, said two Iranian missiles fell near and inside the eastern town of Mayadeen, an Islamic State stronghold. He said there were no casualties from the strikes. ISIS did not immediately acknowledge the strikes.