The UN-backed Syrian peace talks continued in Geneva on Tuesday without the regime delegation after the Syrian government team left last week without confirming it would return.
This eighth round of peace talks will continue to December 14. Negotiations resumed on Wednesday without the regime delegation.
Opinion: What is the price of peace in Syria? "They are.in a hurry to find a solution".
The Britain-based Syrian Observatory for Human Rights says 21 civilians were killed, among them 9 children.
This verified image made from amateur video shows an explosion in the village of Kfar Derian, Syria. The report quoted a Foreign Ministry source.
The delegation returned to Damascus to "consult and refresh", but chief negotiator Bashar al-Ja'afari initially threatened not to come back, which the opposition said would be "an embarrassment to Russia".
"This refusal highlights the obstruction strategy of the political process carried out by the Damascus regime, which is responsible for the absence of progress in the negotiations", he added.
The opposition negotiating team arrived at the United Nations offices in Geneva on Wednesday morning to resume talks with de Mistura, who declined to comment late on Tuesday when asked about the absence of Ja'afari's negotiators.
He also said that Russian Federation, as one of Assad's main supporters, needed to assume its responsibilities so that the Syrian government finally entered the negotiations.
Syria's opposition has insisted that Mr Al Assad must step down, but his negotiators have refused to discuss the issue and the regime's recent successes on the battlefield have strengthened his hand.
De Mistura has tried to maintain an upbeat note on the talks, and on Friday he published a document suggesting 12 principles for a future Syria that he suggested the two sides could agree, including that the country "shall be democratic and non-sectarian".
UN Syria envoy Staffan de Mistura announced shortly after the talks began that they would be extended by two weeks. Despite this, he said that face-to-face contact was less important than the substance of talks, and that the atmosphere was "professional and serious" on both sides.