The United States for the first time held Russia responsible for the deaths of dozens of Turkish troops in Syria and announced new sanctions on Syrian officials.
At least 34 Turkish soldiers were killed in Idlib last month in an airstrike which Ankara blames on Syrian regime forces. Turkey then made a new ceasefire deal with Moscow.
"We believe Russia has killed dozens of Turkish military personnel in the course of their military operation," said US Secretary of State Mike Pompeo while addressing reporters.
"We stand with our NATO ally Turkey and will consider additional measures that support Turkey at the end of the violence," he added.
Analysts widely doubted that Syrian regime rundown air force could effectively hit Turkish forces but until now the Washington had steered clear of blaming Russia.
Idlib has been under deadly regime and Russian fire since December. The offensive has caused a new humanitarian crisis in Syria's brutal nine-year civil war with close to one million people fleeing to border with Turkey.
Pompeo announced new sanctions against Syrian Defense 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.
"The Department of State is imposing sanctions under Executive Order 13894 on Minister of Defense Lieutenant General Ali Abdullah Ayoub for his deliberate actions since December 2019 to prevent a ceasefire from taking hold in northern Syria," Pompeo said in a statement.
"This obstruction resulted in almost a million people being displaced and in dire need of humanitarian aid in the midst of a cold winter in Idlib," the statement added.
The statement also accused the regime of Bashar Assad, backed by Russia, for continuous bombardments destroying schools and hospitals and killing civilians, including medical professionals and first responders who risk their lives to save others.
