Russia has threatened to shoot down any coalition jets flying west of the Euphrates after the US shot down a Syrian warplane. Meanwhile Iran has launched a ballistic missile attack in support of Syria.
Russia threatened on Monday to "target" coalition warplanes and drones flying west of the Euphrates River and said it was suspending a hotline designed to prevent accidents after the US shot down a Syrian jet.
A US fighter jet shot down a Syrian SU-22 on Sunday after the jet had "dropped bombs" near US-backed forces fighting the so-called "Islamic State" (IS) the Pentagon said. Fighters with the Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab militias leading an offensive against the IS stronghold of Raqqa, were bombed near the coalition-controlled Tabqa, it said.
Syria and Russia called the US action a violation of Syria's sovereignty and an act of "aggression."
"Repeated military actions by US aircraft against the lawful armed forces of a United Nations member state, under the guise of a 'fight against terrorism,' are a profound violation of international law and, in fact, military aggression against the Syrian Arab Republic," the Russian Defense Ministry said in a statement.
Russia has aircraft, ground forces and advanced anti-air defenses in Syria in support of the Assad regime.
Speaking in Washington, US General Joe Dunford, chairman of the Joint Chiefs of Staff, said the US would work through diplomatic and military channels to reestablish the deconfliction hotline with Russia.
Meanwhile, Pentagon spokesman, Major Adian Rankine Galloway, said the US had taken "prudent measures to re-position aircraft over Syria" to avoid conflict.
"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," it said in a statement.
Coalition ally Australia said on Tuesday it would temporarily suspend its air strikes into Syria "as a precautionary measure," in response to Russia's threats. Australia has among the highest number of airstrikes in Syria and Iraq.
The Netherlands said it would still send a KDC-10 military refueling plane to Kuwait to assist US-led airstrikes.
Blow for the region
"It's really a blow again," Donald Jensen, a senior fellow at the Center for European Policy Analysis and a former US diplomat said, referring to a previous incident when Russia had also cut off contact.
"This is reflective of a deeper problem, that the strategy and goals of Russia and the US in Syria are incompatible. They've tried to paper over the problem, but it remains."
Charles Kupchan, senior fellow at the Council on Foreign Relations and a professor at Georgetown University, said the tension arises from competing efforts to defeat IS and become a power broker in the region.
"There is a race going on in Syria to fill the gap left by retreating IS forces," he told Deutsche Welle. But he said communication would likely continue regardless.
"I'm guessing this will be a temporary pause. It may also be that folks on the ground will continue to talk on an ad-hoc basis, if only because of the need to do so for purposes of deconfliction."
The US-backed SDF group largely controls territory east of the Euphrates after inflicting a series of defeats on IS over the past two years. It controls two pockets west of the Euphrates near Manbij and further south at Tabqa.
West of the Euphrates, IS had controlled large areas before suffering a series of setbacks in recent months as pro-regime forces recaptured territory.
Iranian show of force
The jet downing came as Iran launched mid-range ballistic missiles from the western provinces of Kermanshah and Kurdistan, targeting what it said were IS militants near the eastern Syrian city of Deir el-Zour, some 700 kilometers (435 miles) away.
Iran's powerful Revolutionary Guard, which has troops in Syria supporting the Assad regime, said six Zolfaghar missiles targeted an IS command center in response to this month's IS terror attacks in Tehran.
But the missile launch was also a show of force directed at the US and Saudi Arabia, sending the signal Tehran could upset US and Saudi interests across the Middle East.
"The Saudis and Americans are especially receivers of this message," Revolutionary Guard General Ramazan Sharif told state television in a telephone interview. "Obviously and clearly, some reactionary countries of the region, especially Saudi Arabia, had announced that they are trying to bring insecurity into Iran."
The US jet downing and Iranian missile strike mark a major escalation in the Syrian conflict, as a multitude of forces maneuver to control the Iraq-Syria border as IS is on the back foot in the Euphrates River valley and eastern desert.
The US has already attacked pro-regime forces three times in the last month and shot down a Syrian or Iranian drone in what Central Command said was in response to pro-regime forces threatening US-backed rebels at a base in al-Tanf, a key transit point on the Syrian-Iraqi border.
The US says the force at al-Tanf is key to pushing north against IS as another the SDF assaults IS-held Raqqa.
But the eastern desert region is highly contested, raising the possibility of mistakes or miscalculations that could quickly escalate between multiple armed actors on the ground.
The Assad regime has not controlled Syria's border with Iraq for three years, and is pushing to take control of the border before US-backed forces converge from the north and south.
On the opposite side of the border in Iraq, Iran-backed Shiite militia known as Popular Mobilization Units have pushed back IS and secured large sections of the border. Shiite militia leaders in Iraq have suggested they may next move into Syria.
For Iran and Hezbollah, both of which back the Assad regime, controlling the border would create a land corridor running from Tehran to Beirut, something the United States is seeking to block.
Meanwhile, the Assad regime has held out in Deir el-Zour, which has been surrounded by IS for several years. The regime seeks to punch out of Deir el-Zour to control surrounding oil wells as well as connect the city to government-controlled areas further west.
aw/rg (AFP, dpa, Reuters, AP) With additional reporting by Chase Winter and Maya Shwayder