The UN's Syria envoy called on the rival sides to meet their historic responsibility and seek a deal to end the six-year conflict ahead of a full day of talks set for Friday (February 24th), AFP reported.

The negotiations at the UN in Geneva formally opened Thursday with envoy Staffan de Mistura dampening hopes of a breakthrough.

"I am not expecting miracles," he said at a welcoming ceremony, while warning of dire consequences if the talks "fail again".

Regime negotiators are meeting the opposition for a fourth round of UN-brokered talks, clouded by persistent violence and deadlock over the country's political future.

De Mistura said he would hold bilateral meetings on Friday devoted largely to procedure. He said his "dream" was to have one opposition grouping but noted that key divisions persist.

The ground -- both in territory and diplomatically -- has shifted since the last UN-sponsored talks to end the conflict, with the opposition now in a significantly weaker position following the regime's capture of Aleppo.

While acknowledging the major obstacles, de Mistura asked the sides to "work together".

"The Syrian people desperately want an end to this conflict and you all know it... they are awaiting for a relief from... suffering and dream of a new road out of this nightmare," he added.