Negotiations in Geneva aimed at ending Syria's devastating civil war have started in a "professional and serious" atmosphere and will be extended another two weeks, the U.N. mediator said Thursday. The meetings, which began Wednesday, were initially expected to last only two to three days, United Nations special envoy Staffan de Mistura told reporters.

"The atmosphere... compared to the past was professional and serious on both sides," he said, adding that the talks were now expected to last until Dec. 15, as reported by AFP. De Mistura said they would take a break over the weekend and would return to Geneva to continue the negotiations next Tuesday.

Syrian regime negotiator Bashar al-Ja'afari said on Friday his team was pulling out of U.N.-led talks in Geneva and might not return next week, citing a statement by the opposition at their "Riyadh 2" conference last month that Bashar al-Assad could play no role in an transition period, according to Reuters.

"As long as the other side sticks to the language of Riyadh 2, there will be no progress," Ja'afari told reporters after a morning of talks, adding that the government in Damascus would decide if his delegation would return next week.