Syria's Kurds have released 30 Syrians suspected of affiliation to the "Islamic State of Iraq and Syria" (ISIS) after guarantees from tribal leaders, a spokesman said Tuesday (January 7th), adding more should be freed soon.

The Syrian Democratic Forces (SDF) hold thousands of suspected ISIS fighters after years of leading the US-backed fight in the country against the extremist group.

The suspects are mostly Syrian and Iraqi, but they also include hundreds of foreigners.

A spokesman for the Kurdish authorities in north-eastern Syria said tribal leaders had requested that 300 Syrian detainees be freed, and that 30 were approved and released on January 5th.

"Those being released have no blood on their hands and have not been proven guilty of any crime," the Kurdish region's foreign affairs spokesman Kamal Akef told AFP.

They might have joined the extremist group to earn a living or because they were forced to, he said.

They "are being released with guarantees from the tribal leaders", the official added, without specifying what guarantees are involved.

"In the coming days, some more will be released in waves" to return to the provinces of Deir Ezzor and al-Raqa in the east and north of Syria, he said.

Backed by airstrikes by the international coalition, the SDF in March last year expelled ISIS from their last patch of territory in eastern Syria.

Earlier that month, nearly 300 Syrian men were freed after tribal chiefs lobbied for their release including in al-Raqa, the Kurdish authorities said.

At the time, the Syrian Observatory for Human Rights said it was not the first release of ISIS-linked prisoners, but the number was particularly large.

The Kurds have also said they were releasing hundreds of suspected ISIS-linked women and children from overcrowded camps for the displaced.