Low-ranking fighters are pardoned as they do not have blood on their hands, the Raqqa Civil Council says.
A US-backed Syrian coalition of Kurdish and Arab groups has captured a western district of Raqqa, the de-facto capital of the Islamic State of Iraq and Levant (ISIL) group.
The Syrian Democratic Forces (SDF) took the al-Qadisia district, they said.
The progress came after three days of intense fighting, the group said in a statement posted on social media.
After launching an assault three months ago, the SDF has pushed ISIL from large areas of northern Syria over the past 18 months.
Turkey-backed Syrian opposition fighters have also taken territory from it and the Syrian army has this year advanced against it in desert areas.
The US-backed coalition has supported SDF advances against ISIL throughout the Raqqa campaign with artillery and air strikes, including some against the group’s leaders.
However, coalition air raids have also caused large numbers of civilian casualties, according to the UK-based Syrian Observatory for Human Rights.
It said on Saturday that coalition air strikes in and around Raqqa had killed nearly 700 civilians so far this year.
In a separate development, Turkish President Recep Tayyip Erdogan said on Sunday that countries that promised to get back weapons supplied to the Kurdish YPG armed group in northern Syria were trying to trick Turkey and would eventually realise their mistake.
Turkey was infuriated by a US decision last month to arm the YPG, which the US sees as a vital ally in the battle against ISIL.
Turkey considers it an extension of the outlawed Kurdistan Workers Party (PKK).
The PKK, designated a “terrorist” group by the US, the EU and Turkey, has been waging an insurgency in southeastern Turkey since the 1980s.
Turkish defence ministry sources said on Thursday that the US had pledged that weapons provided to the YPG would be taken back once ISIL was defeated.
Erdogan dismissed those assurances, saying Turkey’s friends and allies were cooperating with “terrorists”.
“The ones who think they are tricking Turkey by saying they are going to get back the weapons that are being given to this terrorist organisation will realise that they are making a mistake eventually,” he said.
“But it will be too late for them,” he added, saying that if violence spilled over Syria’s border into Turkey,
Turkey would hold to account anyone who supplied arms to the YPG.
US President Donald Trump decided to arm the YPG fighters despite Turkey’s objections and a direct appeal from Erdogan at a White House meeting last month.