"The shooting down of a Syrian Air Force jet in Syria's airspace is a cynical violation of Syria's sovereignty," Russia's Defense Ministry said Monday, according to a report from the state-owned news agency TASS.
"Any aircraft, including planes and drones of the international coalition, detected in the operation areas west of the Euphrates River by the Russian air forces will be followed by Russian ground-based air defense and air defense aircraft as air targets."
Also, Moscow said it terminated the military hotline previously used by U.S. and Russian forces in the region. The so-called "de-confliction" hotline has been used as a safety mechanism and information sharing link between the two sides.
Playing out in the background, however, are concerns that a Russian strike on a U.S. military aircraft could provoke a wider conflict and raise the stakes to dangerous levels.
"I would be very surprised if they willfully shot down a U.S. plane that came to bomb some al Qaeda target west of the Euphrates River," said Michael O'Hanlon, a senior fellow in foreign policy at the Brookings Institution, a think-tank in Washington. "On the other hand, if we're coming after Assad's forces with some big armada, maybe they want us to be a little nervous that maybe we'd get shot at."
O'Hanlon added, "They're trying to create some deterrents, or some limits, on our involvement in escalation. And it's consistent with the way this thing has been going back and forth for a while."
Then again, some analysts believe the Russians are issuing empty threats because they can't afford the consequences of doing anything more.
"A lot of of this is saber rattling and bluster from the Russians who are clearly unnerved actually by recent U.S. actions," said Gardiner.
Gardiner, a former aide to the late British Prime Minister Margaret Thatcher, said it's significant that when a plane from NATO-member Turkey shot down a Russian fighter in 2015 there was rhetoric from Moscow but not a whole lot of action.
"The Russians seem to be very wary of taking on NATO aircraft," he said. "It's not in the interest of Russia to take on the United States because the United States has vastly superior air power in the Middle East. They would not win any conflict like this."
Defense analysts note that the U.S. military has special operations personnel and some conventional forces alongside the SDF. The U.S. had more than 500 special forces in Syria at the end of the Obama administration and that figure is believed to be rising in the Trump administration but the Pentagon declined to provide an exact number.
The Russians are said to have several thousand troops in Syria — largely on bases they operate — and well over 40 military aircraft in the Syrian region, ranging from tactical bombers and fighter jets to attack helicopters. They also have used some of their most advanced fighter jets and attack copters for the first time in combat during the Syrian conflict.
In Moscow, Putin said last week during a call-in show broadcast nationally that his armed forces have gained "precious" experience in Syria because it's allowed them to see firsthand how some of Russia's newest weapon systems are used in real combat situations. Also, Putin said the experience in Syria has allowed "some fine-tuning" of the new weapons to make them better, according to a transcript of the show provided by the Kremlin.
Meantime, the downing of the Syrian government jet Sunday follows Trump's decision to launch a Tomahawk missile strike in April against an Assad military base the U.S. believes had a role in a chemical-weapons attack on civilians. The April chemical attack in Khan Sheikhoun, a rebel-held town in northern Syria, claimed at least 70 lives.
Under President Donald Trump, the U.S. has stepped up the use of firepower to respond the Damascus regime when it gets out of line. During the campaign, Trump was critical of then-President Barack Obama for saying he would establish a "red line" against the use of chemical weapons in Syria and then not acting when the Assad regime used them.
"This [Syrian jet downing] is a very different incident and does signal an escalation in the conflict and U.S. involvement there," said Melissa Dalton, a Middle East expert and senior fellow and deputy director of the International Security Program at the Center for Strategic and International Studies, a Washington think-tank.
That said, Dalton believes Washington and Moscow don't want to see a situation that could spiral out of control and likely are working behind the scenes to reduce the chance of any miscalculation.
In a statement, a Pentagon spokesman said the Syrian jet downing was "in accordance with rules of engagement and in collective self-defense of coalition partnered forces."
Added the Pentagon spokesman, "The coalition presence in Syria addresses the imminent threat ISIS in Syria poses globally. The demonstrated hostile intent and actions of pro-regime forces toward coalition and partner forces in Syria conducting legitimate counter-ISIS operations will not be tolerated."
"The United States is willing to stick up for these partners to such an extent and put its own forces at risk and incur some of the escalation risks," said Dalton.
According to the Pentagon, following pro-Syrian government forces on the ground attacking the coalition-backed SDF, the U.S. alerted its Russian counterparts via the military hotline "to de-escalate the situation and stop the firing." Yet it said the Syrian regime's military jet still dropped bombs near the SDF fighters so the U.S. downed the aircraft.
The Russians, though, dispute that the U.S.-backed coalition command used the communication line. They also insist the Syrian plane was to provide ground cover for Syrian ground forces that were moving against ISIS.