A suspected US-led coalition air strike in Syria's eastern province of Deir Al Zor killed 23 civilians on Monday, the Syrian Observatory for Human Rights said.
The Observatory said jets, thought to belong to the US-led coalition fighting Daesh in Syria, hit the town of Al Bukamal near the border with Iraq. The warplanes struck near a residential area and a mosque, wounding dozens more, according to the Observatory.
Daesh holds most of Deir Al Zor province, apart from an enclave at the centre and a nearby air base that the Syrian regime controls. The province links territory Daesh controls in Syria and Iraq.
Earlier in May, the US military claimed that coalition air strikes in Syria and Iraq had "unintentionally" killed 352 civilians since its offensive against Daesh in 2014.
UN Syria talks
The latest incident comes ahead of a new round of Syrian peace talks that open in Geneva on Tuesday, reconvened by United Nations mediator Staffan de Mistura between the Syrian regime and opposition.
The talks, in which the two sides are not meeting face to face, will take place from May 16 to 19.
De Mistura voiced hope that the agreement reached two weeks ago in Astana among Russia, Iran and Turkey to set up "de-escalation zones" in Syria will be implemented in full.
Opposition delegations have confirmed their attendance and will arrive in Geneva later on Monday.
Russia says no interest in arming "Kurdish formations" in Syria
Meanwhile, on Monday, Russian President, Vladimir Putin, on the sidelines of the China "Silk Road" project in Beijing, said that Moscow saw no need to arm groups fighting in Syria, but would maintain working contacts with them.
"Unlike other countries, we are not announcing any arms deliveries to Kurdish formations; we don't believe we need to start such work," said Putin.
The Russian leader's comments appeared to be aimed at the US decision to supply the YPG as it advances towards Daesh's Syrian stronghold of Raqqa.
Turkey, a crucial partner in the US-led coalition against Daesh, considers the YPG as Syrian extension of the PKK. The PKK has waged a three-decade insurgency in southeast Turkey, leading to tens of thousands of deaths. It has been designated a terrorist group by Turkey, the US and the EU.