The developments added to already-soaring regional tensions and reflect the intensifying rivalry among the major players in Syria's civil war that could spiral out of control just as the fight against the Islamic State group in its stronghold of Raqqa is gaining ground.
Russia, a key ally of Syrian President Bashar Assad, called on the U.S. military to provide a full accounting as to why it decided to shoot down the Syrian Su-22 bomber.
The U.S. military confirmed that one of its F-18 Super Hornets shot down a Syrian jet that had dropped bombs near the U.S. partner forces SDF. Those forces, which are aligned with the U.S. in the campaign against the Islamic State group, warned Syrian government troops to stop their attacks or face retaliation.
The Russian Defense Ministry said in a statement that as of Monday, all coalition jets and drones flying west of the Euphrates River will be tracked as potential targets.
Areas of northern Syria west of the Euphrates were controlled by IS before Syrian government forces captured most of them in recent months. The Russians, who have been providing air cover for Assad's forces since 2015, 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.
It was the second time Russia suspended a hotline intended to minimize incidents with the U.S. in Syrian airspace. In April, Russia briefly suspended cooperation after the U.S. military fired 59 missiles at a Syrian air base following a chemical weapons attack that Washington blamed on the Assad government.
Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, said Washington is working to re-establish communications aimed at avoiding mishaps involving U.S. and Russian air operations in Syria.
Speaking in Washington, the top U.S. military officer said the two sides discussed the matter as recently as Monday morning but that further talks are needed.
Viktor Ozerov, chairman of the defense and security committee at the upper chamber of Russian parliament, described his Defense 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."
Iran said the missile strike by its powerful Revolutionary Guard hit Syria's eastern city of Deir el-Zour on Sunday night and was in retaliation for two attacks in Tehran earlier this month that killed 17 people and were claimed by the Islamic State group.
It 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 Assad. The muscle-flexing comes amid the worsening of a long-running feud between Shiite powerhouse Iran and Saudi Arabia, with supports Syrian rebels and has led recent efforts to isolate the Gulf nation of Qatar.
"The Saudis and Americans are especially receivers of this message," Gen. Ramazan Sharif of the Revolutionary Guard told Iranian state TV in an interview.
It also raised questions about how U.S. President Donald Trump's administration, which had previously put Iran "on notice" for its ballistic missile tests, will respond. Israel also is concerned about Iran's missiles and has deployed a multilayered missile-defense system.