A USA airstrike struck pro-Syrian government forces that the coalition said posed a threat to American troops and allied rebels operating near the border with Jordan on Thursday, the first such close confrontation between US forces and fighters backing President Bashar Assad.
The de-conflicted zone, agreed between the U.S. and Russian Federation, is located around Tanf, where the borders of Jordan, Syria and Iraq meet. The officials say the strike was a defensive move to protect the USA allies.
Tuesday's start of talks in Geneva were overshadowed by US accusations that the government in Damascus carried out mass killings at a prison near the Syrian capital and then burned the victims' bodies in a crematorium.
The officials weren't authorized to speak publicly on the matter and demanded anonymity. The negotiations are focused on four main issues - anti-terrorism action, a new constitution, elections and governance in a post-war Syria.
The IS-linked Aamaq news agency said the militants captured villages of Aqareb al-Safi and Mabouja.
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The Observatory also said that 52 people were killed in the fighting, with the dead including 15 civilians, 27 Syrian soldiers and 10 unidentified people.
Regarding Friday's meeting with De Mistura, Hariri said that they are going to raise two issues with the United Nations that are "the destructive and devil role of Iran" in Syria and the issue of thousands of Syrians who are detained by the Syrian regime.
The team would begin work immediately on "specific options for constitutional drafting" in order to "prevent a constitutional or legal vacuum at any point during the political transition process being negotiated", according to a copy of the document seen by AFP.
Opposition spokesman Salem Meslet says talks are focusing on "the constitutional ground for a political transition process based on the formation of a body with full executive powers".
State TV said two people were wounded in IS shelling on the nearby town of Salamiyeh.