A US-led air attack in eastern Syria that killed dozens of government troops fighting so-called Islamic State has endangered an already shaky ceasefire deal, Russia says.
Russia says at least 62 Syrian troops were killed in the Deir al-Zour strikes on Saturday.
The US has expressed "regret" for the "unintentional loss of life".
Syria says it has lost a warplane in the area, which so-called Islamic State (IS) said it had shot down.
The BBC's Lina Sinjab, reporting from Beirut, says it is rare for the Syrian government to acknowledge losses caused by IS, and the announcement may be intended to draw attention to the consequences of the US-led air strike.