A USA warplane on Sunday shot down a pro-Assad military aircraft that dropped bombs near American-backed fighters.
A top Russian diplomat on Monday condemned the United States for shooting down a Syrian Air Force fighter jet the previous day as an act of "aggression", while US -backed opposition forces on the ground warned Syrian government troops to stop their attacks or face retaliation.
But in a warning that the USA would protect its partners, Spicer said: "The Syrian regime and others in the region need to understand that we will retain the right of self-defense of coalition forces aligned against ISIS".
The Russians also at least temporarily halted use of the hotline aimed at preventing accidental mid-air conflicts - just as it did after the April U.S. missile strikes on a Syrian airfield in response to yet another Assad chemical weapons attack on his people.
Russian Federation has been providing air cover for Syrian president Bashar Assad since 2015 while the coalition has been bombing Islamic State (IS) targets since 2014, sometimes alongside regime forces.
The U.S. says it downed a Syrian military jet on Sunday because the aircraft had dropped bombs near the U.S. -backed Syrian Democratic Forces conducting operations against the Islamic State group.
At a luncheon in Washington, Chairman of the Joint Chiefs of Staff Joseph Dunford said that the United States would work both by diplomatic and military channels to re-establish the line of communications to avoid confrontations and reduce tensions, adding that there are still ways to avoid clashes although U.S. and coalition forces can fully defend themselves, Efe news reported.
The Russian defense ministry says in a statement that, starting Monday, it will track all jets and drones of the US -led coalition west of the Euphrates and treat them as targets.
The US military statement said it acted in "collective self-defence" of its partner forces and the US did not seek a fight with the Syrian Government or its Russian supporters.
Damascus and its ally Moscow condemned the "aggression".
And that got the attention of Syrian President Bashar Assad's chief protector, Russia.
Earlier Russian Deputy Foreign Minister Sergei Ryabkov compared the downing to "helping the terrorists that the USA is fighting against".
Ryabkov asks: "What is this, if not an act of aggression?"