Kurdish fighters in Syria say they have captured eight foreign jihadists including a US teen as they continue to decimate Islamic State's diminishing forces.

The eight, detained on Sunday and Monday, include a 16-year-old American as well as a German and a Russian, the Kurdish People's Protection Units (YPG) said in a statement.

The Kurdish force say the group were plotting an attack on civilians.

They identified the alleged terrorists as Soulay Noah Su (aka Abu Souleiman al-Amriki), a 16-year-old American and Lucas Glass (aka Abu Ibrahim al-Almani), a 31-year-old German.

A file photo of members of the Syrian Kurdish People's Defence (YPG) raising a flag during a victory celebration in Syria, the Kurds - alongside the SDF and US air strikes - have now battered ISIS into an ever diminishing corridor of the country

The YPG released this image of their prisoners; the 16-year-old American, Soulay Noah Su (aka Abu Souleiman al-Amriki), is top row second from the right; the German national Lucas Glass (aka Abu Ibrahim al-Almani) is bottom row far right

A map showing the south east of Syria, where ISIS have been pushed into a narrow corridor east of the Euphrates where one of their last strongholds, Hajin, was taken by US-backed militia on December 14; a pocket of desert territory to the west still remains under jihadist control

They listed the names of the fighters along with their Arabic aliases alongside their mugshots, according to Rudaw.

The news comes as Syrian Democratic Forces - who fight alongside the YPG - announced on Monday they had detained two Americans among five foreign jihadists.

The two Americans, two Pakistanis and an Irishman were part of a cell planning an attack on civilians fleeing the jihadist group's last bastion, the SDF said.

The force, which receives key support in the air and on the ground from the US military, said in a statement that the jihadists were captured on December 30.

The Kurds in northeastern Syria say they hold around 1,000 foreign jihadist fighters, as well as 550 foreign women and 1,200 children who lived with them.

The SDF, backed by coalition air strikes, has achieved major gains since the launch four months ago of an offensive to root out IS from the last rump of the once-sprawling 'caliphate' it proclaimed in 2014.

They have cornered ISIS into a shrinking strip of land east of Syria's Euphrates River around the town of Hajin - one of their largest remaining strongholds which was captured last month.

The jihadists are clinging bitterly to a handful of villages in this river valley and have pledged to fight to the death.

The largest ones are Sousa and Bahgouz, following the capture on Saturday of Al-Shaafa, according to the Syrian Observatory of Human Rights war monitor.