According to a USA defense official, a convoy of 20 pro-regime vehicles was headed toward al-Tanf, a military base on the Syrian-Jordanian border, on Wednesday night.
In September, however, waves of US -led coalition aircraft carried out a series of air strikes against the Syrian army positions near the Deir ez-Zor airport, killing 62 soldiers and wounding approximately one hundred others in what American commanders said was a case of mistaken identity.
US-led warplanes carried out Thursday's (May 18) strike in the east of the country against a convoy of pro-government forces headed towards a remote coalition garrison near the border with Jordan.
In an email to The Associated Press, the USA -led coalition's press office said that Syrian pro-government "forces must now withdraw outside of the established de-confliction zone to avert further Coalition action and remove the threat to our forces".
At the time of the strike, pro-Assad forces "were advancing well inside" the de-escalation zone, the coalition said in a Thursday statement.
The statement says coalition forces have been operating in the area "for many months training and advising vetted partner forces" in the battle against IS.
The strike showed American commanders are willing to use force to maintain de facto safe zones in the country's east, where US forces are training local militias to battle the Islamic State and provide security in liberated regions.
Coalition officials said Russian forces had apparently unsuccessfully tried to dissuade the pro-regime movement south.
Russian Federation has condemned a series of U.S. coalition airstrikes on a military convoy allied with President Bashar al-Assad in Syria as "totally unacceptable". The officials say the strike was a defensive move to protect the US allies.
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The unnamed official's comments were carried by Syrian state TV a day after the strike.
The strike took place in At Tanf near Syria's border with Jordan and Iraq, the official said. "The agreed upon de-confliction zone agreement remains in effect".
Syria's main opposition High Negotiations Committee leader Nasr al-Hariri and UN Special Envoy of the Secretary-General for Syria Staffan de Mistura during a meeting in Switzerland..
Earlier on Thursday, Syrian state media and SOHR said ISIL fighters attacked villages near the only useable road that links the government-controlled cities of Aleppo and Homs, killing many residents.
Syrian Observatory for Human Rights, a UK-based monitor, reported eight killed, "most of them non-Syrian".
"Again and again we see the desire and confirmation of this desire as for the deployment of oppositionists and some extremists, including from Jabhat al-Nusra, to fight against the legitimate government of Syria", Lavrov said commenting on the United States strike.
Thursday's attack comes as government forces are on the offensive against the extremists in other parts of Syria. It said troops launched a counteroffensive under the cover of airstrikes.
The IS-linked Aamaq news agency said the group captured Aqareb al-Safi and Mabouja.