Syria's warring sides entered a final day of UN-backed talks on Friday with little sign of progress towards ending the conflict and with negotiations overshadowed by swelling tensions on the ground.
The sixth round of talks in Geneva was the latest effort to bring a political solution to the war, which has displaced millions and claimed more than 320,000 lives since 2011.
But the sluggish negotiations have been eclipsed by US accusations of new regime atrocities at a prison and the bombing of a pro-government convoy by US-led coalition warplanes.
The international alliance -- which usually strikes IS-- for the first time hit regime-allied forces as they headed toward a remote coalition garrison near the southern border with Jordan, US officials said Thursday.
The strike was sharply condemned by Syria's government.
Speaking to journalists after his final meeting with UN special envoy Staffan de Mistura in Geneva, government-delegation chief Bashar al-Jaafari slammed the "massacre caused by the American aggression on our country yesterday."
The opposition High Negotiations Committee (HNC) spokesman Yehya al-Aridi meanwhile welcomed the strike, hailing "robust action against the foreign forces who have turned Syria into a killing field."
The US-led coalition bombing the Islamic State (IS) militants in Syria and Iraq said it had struck pro-regime forces in southern Syria "that posed a threat to US and partner forces."
Syrian state media gave no precise toll in the attack, but the Syrian Observatory for Human Rights, a Britain-based monitor, reported eight killed, "most of them non-Syrian."
The event was not expected to ease de Mistura's task in Geneva, where years of diplomatic efforts have failed to produce any concrete momentum towards ending the devastating conflict.
The HNC has continued to call for Syrian President Bashar al-Assad's ouster as part of a political transition -- a demand the regime has brushed off as a non-starter.
The current round of talks, which opened Tuesday, were expected to cover four separate topics: the constitution, governance, elections, and combatting "terrorism," but Jaafari said none of the subjects had been discussed.
Instead, the sole tangible product has been an agreement to set up technical committees to discuss constitutional issues with the United Nations.
"This is the only result from this round," Jaafari said.
The HNC was expected at the UN Friday afternoon for its last session with de Mistura.
The umbrella group has struggled with deep divisions, which came to the fore late Thursday when several rebel groups suspended their participation in the delegation, citing dissatisfaction with its negotiation strategy.
But by morning, the HNC said its delegation would be taking part in the meeting with the UN envoy "in full".
Syria's conflict erupted in March 2011 with widespread demonstrations against Assad, but has since evolved into a bloody war that has drawn in world powers on all sides.
Rebel forces have received backing from Gulf states, Turkey, and the West, while the Syrian army is bolstered by Russia, Iran, and fighters from neighbouring Lebanon and Iraq.
Russian deputy foreign minister Gennady Gatilov blasted the coalition strike on regime forces, saying it would take a toll on the political process in Geneva.
"Such actions that were carried out against the Syrian armed forces... this is completely unacceptable, this is a breach of Syrian sovereignty," he was quoted as saying by state-run RIA Novosti in Switzerland.
Gatilov held a series of meetings throughout the week with the government and opposition delegations as well as de Mistura.
Russia, Iran, and Turkey are co-sponsors of a parallel negotiations track in the Kazakh capital Astana.
Earlier this month, they agreed to create four "de-escalation zones" across some of Syria's bloodiest battlegrounds, and observers said de Mistura would be scrambling to match Astana's momentum.
A western diplomat in Geneva said the UN's initiative on constitutional meetings was an effort to "get people into substance."
"This is about the special envoy keeping the Geneva process warm and relevant. There's been a lot of talk about Astana lately, but this is actually the main show for solving the Syrian conflict," the diplomat said.