Activists say at least 18 civilians have been killed in air strikes in the Syrian city of Raqqa, the headquarters of so-called Islamic State.
Dozens more people were injured in the raids on Tuesday, according to the Syrian Observatory for Human Rights.
It said it was not able to determine who carried out the strikes, though another group blamed Russian warplanes.
There were further air strikes on Raqqa on Wednesday, but this time reportedly by the US-led coalition against IS.
There were no immediate reports of casualties from the raids, one of which hit the city council building.
The Syrian Observatory reported that Tuesday’s air strikes in Raqqa targeted several areas of the city, which has been controlled by IS since August 2013.
Six children were among those killed, the UK-based monitoring group said, warning that the death toll was likely to rise because many of the injured were in a critical condition.
The anti-IS activist group, Raqqa is Being Slaughtered Silently (RBSS), listed the names of 18 people, including two children, who it said were killed in three raids by Russian aircraft on Tuesday. Twenty-eight others were wounded, it added.
RBSS said at least one air strike targeted an area popular among foreign fighters – jihadists who have migrated to Syria to fight for IS.
IS militants in Raqqa province and neighbouring Aleppo are under pressure from two separate offensives by Russian-backed Syrian government forces and an alliance of Kurdish and Arab fighters supported by the US.
On Tuesday, the militants reportedly drove back government forces around the town of Tabqa, 40km (25 miles) west of Raqqa, after two days of fierce clashes that left 40 soldiers and allied militiamen dead.
The IS-linked Amaq news agency posted video purportedly showing militants in control of the Thawra oil field, which the government forces recaptured on Sunday.
Fighters from the Kurdish-dominated Syrian Democratic Forces are meanwhile besieging IS-held Manbij, 110km (70 miles) to the north-west.
At least three suicide bombers attacked SDF positions outside the town on Tuesday, according to the Syrian Observatory.
Fighting also continued on Tuesday between government troops and rebel forces around the divided northern city of Aleppo.