Russia, a major backer of Syria's government, has accused the United States of having "compromised" a fragile ceasefire in Syria's Idlib province by launching an attack on the rebels' last remaining bastion.
The US Central Command (CENTCOM) said on Saturday it had hit an al-Qaeda-linked training camp in northern Idlib, targeting leaders that were "responsible for attacks threatening US citizens, partners, and innocent civilians".
The Russian military said on Sunday that the US struck the region "without advance notice to Russia or Turkey", which have troops on the ground in Idlib. It described the attack as "indiscriminate".
The raid caused "great losses and destruction", the Russian defence ministry added in a statement, accusing Washington of having "compromised the ceasefire in the de-escalation zone of Idlib".
Russia intervened in Syria's long-running conflict almost four years ago in support of Syrian President Bashar al-Assad, while Turkey has long backed rebels in Idlib. The two countries cosponsored a de-escalation agreement for Idlib that has been in place since last year but faltered in recent months.
CENTCOM declined to say what kind of weaponry was used in the attack in Idlib, where Russia-backed government forces launched an offensive in late April to seize it from rebels.
The Britain-based Syrian Observatory for Human Rights, a war monitor, said "at least 40 fighters were killed" in what it called a missile attack.
Hurras al-Deen targeted
On July 1, the US said it had carried out a raid on the al-Qaeda-linked Hurras al-Deen armed group in northwest Syria in its first such operation there in two years.
The group and its ally Ansar al-Tawhid operate in the Idlib region and are members of a joint operation room that also includes al-Qaeda's former Syria affiliate, Hay'et Tahrir al-Sham (HTS).
Most of Idlib province and parts of neighbouring Aleppo and Latakia provinces are controlled by HTS.
Separately, Syrian government air raids on Idlib halted early on Saturday, after the government agreed to a Moscow-backed ceasefire following four months of deadly bombardment.
Russia-backed government forces have been pressing an offensive against the major opposition stronghold since late April.
Syrian state news agency SANA on Saturday said the government agreed to the Idlib ceasefire deal, which Russia said aimed "to stabilise the situation" in the anti-government bastion. But the army "reserves the right to respond to violations" by rebel groups, SANA added,
Saturday's truce is the latest attempt to avert a full-blown Syrian offensive, which the United Nations has said would result in one of the worst humanitarian "nightmares" in Syria's eight-year conflict.
Idlib is home to some three million people, nearly half of whom have been displaced from other parts of Syria after being transferred en masse to the region from other areas that fell to al-Assad's forces.
The UN says recent violence has also displaced more than 400,000 people.
The Syrian conflict has killed hundreds of thousands of people and driven millions from their homes since it started with the brutal repression of anti-government protests in 2011.