As the US-backed Syrian force fighting ISIS advances on its last pocket of territory in the Euphrates river Valley, six more suspected foreign fighters have been captured.

“Six foreign jihadists of various nationalities who had actively participated in terrorist activities of the group for years were captured alive in an operation,” the Kurdish-led Syrian Democratic Forces said in a statement on Saturday.

The men are reportedly from Russia, Germany, Turkey, Sweden, Morocco and an unspecified country in Latin America.

The once sprawling "caliphate" declared by ISIS in 2014 has now been reduced to a pocket of land just four kilometres square, the SDF said last week. The militia is now in the final stages of an operation launched over a month ago to retake the last territory still under ISIS control.

More than 36,000 people have fled the “Hajin pocket” since the SDF assault began in early December, according to the Syrian Observatory for Human Rights, a Britain-based war monitor.

Hundreds of civilians continue to flee the area, while ISIS fighters detained others for use as human shields, SDF spokesman Mustafa Bali said.

Syrian people return to their homes after the area was freed from ISIS in Hajin on February 2, 2019. AFP

Among the fleeing civilians last week was a German woman who came to live under ISIS as a 15-year-old. A woman identified by AFP as Leonora, now 19, escaped to a screening centre for the displaced run by the SDF in Baghouz, where she described travelling to Syria two months after converting to Islam.

She became the third wife of a German ISIS fighter named Martin Lemke, bearing him two children in Syria.

Her husband was detained by the SDF after the family fled the fighting. “We gave up together,” she said.

Other escaping ISIS fighters are believed to be among the fleeing civilians, while others are trying to negotiate their escape, SDF spokesman Mr Bali said. “They send messages via smugglers demanding an agreement which will let them flee the area in return for releasing civilians.”

No safe passage would be offered to ISIS fighters, he said. “Operation will go on until the last terrorist is dead.”

The SDF continues to take prisoners, with over 800 suspected foreign fighters and a similar number of local fighters now held in makeshift prisons in northern Syria. As United States President Donald Trump continues mulling a complete withdrawal of troops from Syria, the fate of these prisoners has become an international issue.

Turkey meanwhile announced on Saturday that its soldiers had captured four ISIS suspects in a Turkish town on the Syrian border as they tried to illegally enter the country.

Border units from the Turkish armed forces captured the four "terrorists" on Friday in the Akcakale district of Sanliurfa in southeast Turkey, the country’s defence ministry said on its official Twitter account.

The SDF hopes to recapture the last ISIS territory within days but is already concerned at the prospect of an insurgency in recaptured areas.

“While the operation to clear ISIS in its last pocket in northern Syria goes on, the sleeper cells the group has left behind to carry out insurgency are also being targeted,” it said in a statement on Saturday. “In this sense we will continue to inform public about the terrorists captured in special operations.”