Since then, the United States has repeatedly struck Iranian-backed militia and last week even shot down a drone threatening US-led coalition forces. The official added that the strike was in "self-defense".
The incident occurred in the same vicinity as previous conflicts with pro-regime forces, near the combat outpost in At-Tanf, where a contingent of USA and coalition advisers are supporting a force to fight the Islamic State, also known as ISIS.
Dr Lina Khatib, head of the Middle East and North Africa Program at global affairs think tank Chatham House, told CNN the ADF decision put the USA in a "very hard position".
On Sunday, the United States shot down a Syrian plane that had dropped bombs near its ground-partnered Syrian Democratic Forces (SDF) south of Tabqa about two hours after SDF fighters were wounded in an earlier attack by pro-Syrian regime forces in Ja'Din.
Asked Tuesday whether the hotline was no longer being used, a Pentagon spokesman, Navy Capt. Jeff Davis, said he could not discuss the matter beyond saying the US stands ready to use the hotline. "We want to de-escalate, not escalate", Davis said.
"There's actually a line we have with the Russians that's a de-confliction line, and that line remains open and we remain in conversation with them", Goldfein said.
He says: "We are not the initiator of any conflict and war but we do not tolerate creation of insecurity in our country".
"The deconfliction channel remains active and in use", Pickart said. The Russian Defense Ministry claims that the air-safety hotline was not used and also noted that there were also Russian aircraft in the area when the plane went down. This could out USA and Russian forces in direct conflict.
Tuesday's encounter is the latest development in what's become an increasingly hostile standoff between the USA military and the various forces supporting Syrian President Bashar al-Assad's fight to stay in power.
The downing of the drone came on the same day Australia announced it would temporarily suspended airstrikes by its forces in Syria after a us fighter jet shot down a Syrian jet, and Syrian ally Russian Federation threatened to target planes from the USA -led coalition operating in the skies over Syria.
U.S. officials likely want to avoid seeing things spiral to a point that ultimately could require a bigger ground troop deployment in Syria than intended, said Stevenson, who is now a senior fellow with the International Institute for Strategic Studies.
US forces shot down an Iranian drone in southern Syria on Tuesday, the second such air-to-air encounter this month as opposing forces converge around a key American training garrison near the border with Jordan and Iraq.
The drone was downed just outside the 55 kilometer de-confliction zone, according to the officials.
The drone is an Iranian-made unmanned aerial vehicle (UAV), the statement explained. That's a questionable line of argument, particularly as unlike the recent strikes around al-Tanf, there's no indication that there was a USA troop presence nearby-only Kurdish SDF forces.
In another development, the USA military announced officially that coalition forces had killed the top cleric of so-called Islamic State (IS), Turki al-Binali, in an air strike in Syria last month.
It said the Coalition forces were manning an outpost northeast of At Tanf where it provides training and advice to its partnered forces against the ISIS group. The decision came amid increasing tension between the US and Russian Federation, which warned it would track coalition aircraft in Syria as potential "targets", and halted a military hotline with Washington over the incident.
In another first, Iran fired ballistic missiles at Islamic State targets in eastern Syria, in the province of Deir el-Zour, later on Sunday.
The missiles were apparently in response to an IS-claimed attack on the Iranian parliament earlier this month which killed more than a dozen people.
A Western intelligence source said it was "unquestionably Iranian".
Sharif says the missile launch reflects Iran's "military power" though Iran has no intention of starting another war.
Some Trump appointees saw Iran's missile strike as an illustration of Tehran's regional ambitions, which they have argued make it an existential enemy, according to the three USA officials.