washington/DAMASCUS -  A pro-regime convoy that was struck by US-led warplanes inside Syria this week likely was directed by Iran, Pentagon chief Jim Mattis said Friday.

The strike on the convoy heading toward a coalition garrison near the Jordanian border was “necessitated by offensive movement with offensive capability of what we believe were Iranian-directed forces,” Mattis said, adding he was not sure if there were Iranians on the ground. Thursday’s strike occurred inside an established “deconfliction zone” northwest of the At-Tanf garrison, where British and US commandos have been training and advising local forces fighting the Islamic State group.

Such zones are agreed upon between Russia and the coalition, and are designed to stop either side inadvertently striking the other’s forces on the ground and in the air.

The Pentagon says coalition attempts to stop the convoy from proceeding had included a call to the Russians - who work with the Syrian regime - then a “show of force” in the skies above the vehicles, followed by warning shots. It appeared the convoy had moved into the area against the advice of the Russians, Mattis said.

“It looks like the Russians tried to dissuade them,” he said. The Pentagon has stressed the attack did not signal broader US involvement in Syria’s civil war, but Damascus on Friday condemned the strike on pro-government forces as a “brazen attack” and said it would “not be intimidated.”

Russian Foreign Minister Sergei Lavrov called the strike a violation of Syrian sovereignty and said he was “unaware” that Russia had been given any warning ahead of the strike. An array of regular and irregular forces are battling alongside the government against rebels, including Russian and Iranian soldiers and militants from Iraq and Lebanon’s Hezbollah group.

Thursday’s strike comes in the context of growing tension over which forces will take on IS in Syria’s east.

President Bashar al-Assad’s army is trying to prevent US-backed forces from leading that fight.

Syria on Friday condemned a US-led coalition strike on pro-government forces as a “brazen attack” and said it would “not be intimidated” after the surprise assault.

Russia condemned the strike as “illegitimate” and said it could harm efforts to fight Islamic State group (IS) militants. US-led warplanes carried out Thursday’s strike in the east of the country against a convoy of pro-government forces headed towards a remote coalition garrison near the border with Jordan.

“On Thursday at 16:30, the so-called international coalition attacked one of the Syrian Arab Army’s positions on the Al-Tanf road in the Syrian Badia region, producing a number of martyrs and causing material damage,” a military source told Syrian state media.

“This brazen attack by the so-called international coalition exposes the falseness of its claims to be fighting terrorism,” it added. “The Syrian Arab Army is fighting terrorism on its territory, and no party has the right to determine the course of its operations,” the source said. “The Syrian Arab Army will... not be intimidated by the attempts of the so-called coalition to stop it from performing its sacred duties.”

Syrian ally Russia also denounced the US-led strike. “It is illegitimate, unlawful and yet another gross violation of Syria’s sovereignty,” Russian Foreign Minister Sergei Lavrov said in televised comments while on a trip to Cyprus. In a statement, the US-led coalition said it had struck “pro-regime forces... that posed a threat to US and partner forces.”

The coalition said the strike came after unsuccessful “Russian attempts to dissuade Syrian pro-regime movement” as well as “a coalition aircraft show of force, and the firing of warning shots.”

Lavrov said he was “unaware” that Russia had been given any warning of the strike, and said preliminary information suggested there were civilian casualties. “We are still confirming the details but according to some reports several dozen civilians died as a result of this strike,” he was reported as saying by Russian media.  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”.

A statement from Centcom, the US military command in the Middle East, said two strikes “destroyed two front-end loaders, a piece of construction equipment, a tactical vehicle and a tank”. An array of regular and irregular forces are battling alongside the government against rebels, including Russian and Iranian soldiers and militants from Iraq and Lebanon’s Hezbollah group. Thursday’s strike comes in the context of growing tension over which forces will take on IS in Syria’s east.

President Bashar al-Assad’s army is trying to prevent US-backed forces from leading that fight.

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 militants - 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 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 group 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.