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WASHINGTON (AP) — U.S.-backed Syrian Kurdish fighters arrested dozens of Islamic State militants and rescued four women who were being held chained in tunnels at the massive al-Hol camp in northeastern Syria that houses IS families, the U.S. military said Wednesday.

The operation, which has gone on for two weeks, is part of an ongoing effort to dissolve a major IS network at the camp, which is widely seen as a breeding ground for the next generation of IS extremists.

According to U.S. Central Command, the Kurdish-led Syrian Democratic Forces found the four women in the tunnels on Monday. They had been tortured by IS supporters, said U.S. Army Col. Joe Buccino, a spokesman for Central Command, which provided no other details.

U.S. and coalition forces have been advising and assisting the SDF, including training the fighters.

The al-Hol camp, in Syria’s Hassakeh Province, has long been viewed as a growing problem and there have been a number of military operations and raids there to root out IS threats.

Some 50,000 Syrians and Iraqis are crowded into tents in the fenced-in camp. Nearly 20,000 of them are children; most of the rest are women, the wives and widows of IS fighters. In a separate, heavily guarded section of the camp known as the annex are an additional 2,000 women from 57 other countries — they are considered the most die-hard IS supporters — along with their children, numbering about 8,000.

The camp was initially used to house the families of IS fighters in late 2018 as U.S.-backed Kurdish-led forces recaptured territory in eastern Syria from the militants. In March 2019, they seized the last IS-held villages, ending the “caliphate” that the group declared over large parts of Iraq and Syria in 2014.

The U.S. and other nations have struggled to repatriate the families, but have had only very limited success.

Syrian Kurdish fighters have intensified their inspections at al-Hol in recent months, following a late January attack by IS on a prison in Hassakeh, where some 3,000 militants and juveniles are held.

That attack led to 10 days of fighting between the U.S.-backed fighters and IS militants that left nearly 500 dead before the situation was brought under control. It was the biggest attack by IS since the fall of the group in 2019, and it stoked fears that IS may be staging a comeback.