The United States says it is concerned by reports that Russia is moving more military equipment into Syria to bolster President Bashar al-Assad, with a truce in tatters and peace talks in meltdown.
Asaad Zoubi, chief negotiator for the main Syrian opposition, the High Negotiations Committee (HNC), said all its members would leave the peace talks in Geneva by Friday, with little prospect of a resumption unless the situation on the ground changes radically.
UN special envoy Staffan de Mistura will decide on Friday whether talks to end the five-year war that has killed at least 250,000 people can go on without the HNC, and with combatants accusing each other of breaking a February ceasefire deal.
Two Western diplomats said it looked like de Mistura would continue the talks until next Wednesday, given the late arrival of the Syrian government delegation.
"The HNC stayed the course, including through extended technical discussions on real substance," one said.
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"It's natural that the special envoy may continue discussion with those still in town who have yet to offer any real ideas, to press them to do so."
A second diplomat said some experts from the HNC would remain for technical consultations.
HNC spokesman Salem al-Meslet suggested de Mistura might be better off preparing for the next round of talks and ensuring the government was serious.
A US official told Reuters on Thursday that Russia has been repositioning artillery to northern Syria - a move that may suggest the Syrian government and its allies are preparing another assault on the divided city of Aleppo.
"It's understandable that the opposition felt unable to stay further given sustained regime attacks on Syrian civilians and continuation of siege and starvation tactics," one senior Western diplomat said.
"Those who back the regime need to get a leash on them."
The arrival of Russian reinforcements would risk driving the war into an even higher gear.
Endorsed by the UN Security Council, the Geneva peace talks marked the most serious effort yet to resolve the war, but failed to make any progress with no sign of compromise over the main issue dividing the sides: Assad's future.
Government negotiators say Assad's presidency is non-negotiable.
Underlining confidence in Damascus, a top Assad aide reiterated its view that local truce agreements and "destroying terrorism" were the way towards a political solution.
The opposition wants a political transition without Assad, and says the government has failed to make goodwill measures by releasing detainees and allowing enough aid into opposition-held areas besieged by the military.
