At least 13 people have been killed in suspected US-led coalition air strikes on the ISIL-held city of Raqqa and suspected rocket attacks fired by a Kurdish group fighting ISIL, a monitoring group has said.

Some of the deaths in the northern city on Sunday evening resulted from air strikes blamed on the US-led coalition, the UK-based Syrian Observatory for Human Rights said on Monday.

The death toll also included civilians killed in rocket attacks by the Ghadab al-Furat group (dubbed Wrath of the Euphrates) on Sunday, the Observatory said.

Ghadab al-Furat is a Kurdish group fighting under the Syrian Democratic Forces (SDF). They launched a campaign in October 2016 to retake Raqqa, the de facto capital of ISIL in northern Syria.

The SDF, which includes the powerful Kurdish YPG armed group, said last week it plans to launch the final assault on Raqqa city in early summer.

US Central Command said in a press release on Sunday that it conducted 17 air strikes targeting ISIL in Syria, destroying two ISIL bases in Deir Az Zor and three ISIL headquarters near Raqqa.

It did not mention civilian casualties in its report.

READ MORE: Civilian toll rises in deadliest month in US-led raids

Raqqa is Being Slaughtered Silently, an activist group in Raqqa, said on Sunday that a school was targeted by the US-led coalition in Mansoura, west of Raqqa city.

The school was destroyed in the attack, the group said.

The activists said on Thursday that Raqqa city was targeted with at least 30 coalition air strikes and 80 rocket attacks by the SDF, killing at least 35 civilians in the past 24 hours.

4- Photos show the destruction because of the artillery shelling by #YPG on #Raqqa city"Almror-Alnoor-Alqwatli"neighborhoods yesterday night pic.twitter.com/aaf8qF1fCS — الرقة تذبح بصمت (@Raqqa_SL) May 29, 2017

The SDF has been encircling Raqqa since November.

Earlier this month, its fighters captured Tabqa, a previously ISIL-held town some 50km west of Raqqa, and a strategic dam nearby.

The UN said in a report that on May 14, at least 23 farm workers, including 17 women, were reportedly killed when air strikes hit al-Akershi village in a rural area of eastern Raqqa province.

Other air strikes on two residential areas of the ISIL-controlled city of Abo Kamal in eastern Deir Az Zor province the following day (May 15), reportedly killed at least 59 civilians (including 16 children and 12 women) and injured another 70.

READ MORE: Syrians' suffering persists after returning to former ISIL-held town

The day after that, ISIL fighters are said to have cut the throats of eight men at the sites of the air strikes,

after accusing them of providing coordinates for the strikes.

Earlier in May, the Observatory reported the highest monthly civilian death toll for the coalition's campaign in Syria.

Between April 23 and May 23, 2017, coalition air strikes killed at least 225 civilians in Syria, including dozens of children.

The US military had said coalition air strikes in Iraq and Syria had "unintentionally" killed a total of 352 civilians since 2014.

At least 23,544 civilians have been displaced on May 18-22, the UN said in a press release last week.

The UN High Commissioner for Human Rights Zeid Raad al- Hussein last week urged all states' air forces operating in the country to take much greater care to distinguish between legitimate military targets and civilians.

"The same civilians who are suffering indiscriminate shelling and summary executions by ISIL, are also falling victim to the escalating air strikes, particularly in the northeastern governorates of Raqqa and Deir Az Zor," Zeid said.