The sixth round of United Nations -backed negotiations is the latest drive to bring a political solution to the conflict which has claimed more than 320,000 lives since 2011. The officials say the strike was a defensive move to protect the USA allies.
"This action was taken after apparent Russian attempts to dissuade Syrian pro-regime movement south. were unsuccessful, a coalition aircraft show of force, and the firing of warning shots", the USA -led coalitions aid.
The U.S. officials said the American airstrike hit the pro-Syrian government forces as they were setting up fighting positions in a protected area near Tanf.
A US airstrike launched in Syria in April.
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The officials weren't authorized to speak publicly on the matter and demanded anonymity. The opposition needed time to prepare its response to a "paper" presented by United Nations special envoy Staffan de Mistura on Tuesday, when the sixth round of the talks started.
"We hope to begin seriously the real political process that achieves political transition within the timeline of aUN Security Council resolution", al-Hariri said.
The UN Syria envoy announced Thursday that the Syrian government, major opposition delegations, and the UN had agreed to kick off a series of expert meetings to discuss constitutional issues, one of the main points of the latest round of Syria peace talks.
The envoy's office said his team also met separately on Thursday with opposition experts on constitutional and legal issues.
"Earlier this week the Special Envoy informed the sides that he meant to establish a technical process to address constitutional and legal issues in the intra-Syrian talks", United Nations envoy Staffan de Mistura's Office said in a written statement.