The US for the first time Tuesday (March 17th) held Russia responsible for the deaths of dozens of Turkish troops in Syria as it vowed accountability.
An airstrike in Syria's north-western Idlib province on February 27th killed 34 Turkish soldiers, although Turkey blamed the Syrian regime and reached a new ceasefire deal with Russia.
US Secretary of State Mike Pompeo, announcing new sanctions on Syrian officials, placed blame on Russia, which along with Iran has backed the Syrian regime in its bloody offensive.
"We believe Russia has killed dozens of Turkish military personnel in the course of their military operation," Pompeo said, without naming a specific incident.
"We stand with our NATO ally Turkey and will consider additional measures that support Turkey at the end of the violence," he said.
Sanctions on Syrian minister
Analysts widely doubted that the Syrian regime's rundown air force could effectively hit Turkish forces, but until now the US had steered clear of blaming Russia, mindful of official statements by Ankara.
After the February incident, Turkey killed dozens of Syrian regime troops in retaliation, but President Recep Tayyip Erdogan later negotiated a ceasefire with his counterpart Vladimir Putin that includes joint Russian-Turkish patrols.
Pompeo also announced new sanctions against Syrian defence minister Ali Ayoub, accusing him of destroying an earlier truce through the offensive.
"His deliberate actions since December 2019 have prevented a ceasefire from taking hold inside Syria," Pompeo said.
Under the sanctions, any US assets of Ayoub are frozen and the US can prosecute anyone for financial transactions with him.
US officials had earlier pointed to the deaths of Turkish troops as proof that Ankara should be cautious about building relations with Russia.
Idlib clashes defy truce
Meanwhile, Tuesday clashes in Idlib killed four regime fighters and one opposition fighter, in violation of a truce that went into effect March 6th, the Syrian Observatory for Human Rights said.
Before dawn on the truce's first day, an exchange of fire killed six regime fighters and nine extremists, but belligerents have largely respected it since.
Despite Tuesday's clashes, regime and Russian aircraft remained absent from the skies over the region, Observatory head Rami Abdel Rahman said.
The Russia-backed regime offensive on the Idlib region had killed around 500 civilians and displaced almost one million people since December.
Residents have said they are skeptical about the ceasefire holding, especially as the Syrian regime has repeatedly said it will eventually bring all of Syria back under its control.