Syrian rivals were to resume UN-brokered peace talks in Geneva on Thursday, with escalating violence and deadlock on key issues dimming hopes of a breakthrough.
UN envoy Staffan de Mistura tried to strike an optimistic note when the previous round ended last month.
He insisted government and rebel negotiators had agreed on a "clear agenda" and that "everything is ready" for the talks to move forward.
On the agenda for this round are issues of governance -- a political transition, the constitution and elections, as well as counter-terrorism at the request of Damascus.
But an already-fragile ceasefire has deteriorated further in recent days after rebels and allied jihadists launched two surprise offensives on government positions in Damascus and central Hama province.
Syrian President Bashar al-Assad's air force has responded with raids on rebel targets, further threatening negotiations which have yielded little in four previous rounds.
The UN talks aim to find a solution to end a six-year conflict that has killed more than 320,000 people and displaced millions since it started in 2011 with protests against Assad's regime.