A Syrian warplane was shot down over Raqqa by the United States-led coalition after a Syrian regime SU-22 dropped bombs near Syrian Democratic Force, or SDF, fighters.
Keane's remarks come hours after the Russian Defense Ministry issued a statement saying saying it will treat US -led coalition planes in Syria that fly west of the Euphrates River as targets.
In a statement, the Russian defense ministry said it would regard any US coalition aircraft detected west of the Euphrates river in Syria as legitimate targets.
"The US" repeated combat operations under the guise of "combating terrorism' against the legitimate armed forces of a United Nations member-state are a flagrant violation of global law, in addition to being actual military aggression against the Syrian Arab Republic", the ministry said.
Russian Federation has criticised the U.S. for shooting down a Syrian air force jet, calling it "an act of aggression" and "assistance to terrorists".
"The SDF is getting big-headed", said the pro-Damascus military commander, a non-Syrian who spoke to Reuters on condition of anonymity. The incident raises the risk of a wider, regional conflagration as the U.S. Assad, Russia, and Iran maneuver for control and influence in territory reclaimed from ISIS in eastern Syria.
Also on June 8, a U.S.aircraft shot down a "pro-regime" Unmanned Aerial Vehicle (UAV) that had fired on coalition forces in the same area of southern Syria.
The Syrian army acknowledged that one of its planes had been shot down.
"What is this if it is not an act of aggression or a direct violation of worldwide law?" he said.
In April, the U.S. Navy launched 59 cruise missiles targeting a Syrian airbase from two destroyers in the Mediterranean following a chemical weapons attack by Assad's forces.
The situation throughout Syria remains extremely fraught, with the Assad regime accusing the US-led forces besieging Raqqa of allowing ISIS fighters to escape to the south, where government troops are battling ISIS for control of the city of Deir es-Zor.
While nominally fighting Islamic State of Iraq and Syria (ISIS) forces, the USA shoot-down makes clear that the real aim of American-led operations is the ousting of the Syrian government of President Bashar al-Assad.
The Syrian army backed by Iranian-backed militias has also been in competition in southeastern Syria with US -backed Free Syrian Army (FSA) rebels who are also trying to recapture territory from ISIS.
The action appears to mark a new escalation of the conflict.
It denied the USA had used a communications channel before the SU-22 fighter bomber was downed.
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".
The escalation comes after the USA shot down a Syrian jet that had been dropping bombs near US-backed fighter jets.
In a show of force, Coalition aircraft stopped the initial advance. However, it has also previously received Russian support in the form of embedded troops, air support, weaponry and training, particularly immediately following the downing of a Russian aircraft by Turkey in 2015.