Two Americans and a Briton believed to be the first western miltants backing the YPG were killed in Raqqa province. Separately, a Russian military instructor was killed in Hama province.
Two Americans and a Brit were killed fighting alongside the US-backed coalition as they battle to oust Daesh from its Raqqa stronghold.
The three men were fighting with the YPG – considered by Turkey to be the Syrian branch of PKK – are believed to be the first western militants backing the YPG to have been killed since fighting began in the northern Syrian city against Daesh.
The YPG said that Americans Robert Grodt and Nicholas Warden and Briton Luke Rutter were among six killed on the Raqqa front, without saying where exactly they died.
The YPG is a leading part of the US-backed Syrian Democratic Forces (SDF) which launched an operation earlier this month to capture Raqqa, Daesh's de facto Syrian capital.
Due to its ties with the PKK, which has waged war against the Turkish state since 1984, Turkey considers the YPG to be a terrorist organisation.
Turkey has said that it would retaliate against the YPG if it felt threatened by the group in any way.
On November 5, 2016, the SDF launched an offensive dubbed Wrath of the Euphrates to retake Raqqa, and after months of fighting in the surrounding province, they entered the city in June for the first time.
Daesh overran Raqqa in early 2014, turning the northern city into the de facto Syrian capital of their so-called caliphate.
Russian military instructor killed
The Russian defence ministry said on Wednesday that its military instructor was killed by mortar fire in Hama province.
Nikolay Afanasov was killed by sudden mortar fire on a Syrian regime army camp, the Russian defence ministry said, indicating that he was part of a group of military instructors who were training regime soldiers.
His death raises the number of Russian military personnel killed in combat in Syria to 32, according to ministry figures.
Another soldier committed suicide during the first month of Russia's operation in Syria which began in September 2015.
Thousands trapped in Raqqa
Up to 50,000 civilians remain trapped in the Daesh stronghold of Raqqa, the UN said on Tuesday, warning that supplies of water and other essentials were fast running out.
An estimated 2,500 Daesh militants are defending the city.
"The UN estimates that between 30,000 and 50,000 people remain trapped in Raqqa city," UN Refugee Agency (UNHCR) spokesman Andrej Mahecic said in Geneva, down from some 100,000 people at the end of June.
Mahecic stressed that it was hard to be sure about the numbers given the lack of access to Raqqa for UN agencies.
Trump, Macron meet on Syria
US President Donald Trump will travel to Paris on Wednesday to meet with French President Emmanuel Macron where the two leaders will seek to work together on Syria and countering terrorism.
The US and French officials have said Trump's visit to Paris will allow the leaders to focus on those places where their interests overlap, including resolving the conflict in Syria and combating global terrorism.