Around 50 Islamic State fighters that the U.S. removed from Syria in recent days amidst a Turkish military assault are headed to Iraq, officials there say.

Two Iraqi intelligence officials who spoke to the Associated Press on condition of anonymity Thursday say the transfer of the prisoners is expected to be completed by Friday.

The move comes after Turkey began a military offensive into northern Syria against U.S.-backed Kurdish-led fighters of the Syrian Democratic Forces (SDF), who are holding more than 10,000 ISIS members as prisoners.

There are concerns that with the assault unfolding, the Kurdish-led forces in charge of guarding the detainees won't be able to secure them or would divert forces to fend off against advancing Turkish troops.

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A Turkish army tank is driven to its new position, after was uploaded from a truck, on the Turkish side of the border between Turkey and Syria, in Sanliurfa province, on Tuesday. (AP)

U.S. officials said Wednesday that two British militants believed to be part of an ISIS group that beheaded hostages and were known as "The Beatles" already have been moved out of a detention center in Syria and are in American custody.

Their exact location Thursday was not immediately clear.

The two British men, El Shafee Elsheikh and Alexanda Amon Kotey, along with other British jihadis, allegedly made up the ISIS cell that was nicknamed "The Beatles" because of their English accents.

In 2014 and 2015, the militants held more than 20 Western hostages in Syria and tortured many of them. The group beheaded seven American, British and Japanese journalists, and aid workers and a group of Syrian soldiers, boasting of the butchery in videos released to the world.

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The two British men were captured in January last year in eastern Syria by the Kurdish forces amid the collapse of ISIS. Their detention set off a debate in the U.S. and Europe over how to prosecute their citizens who joined ISIS.

ISIS leader Abu Bakr al-Baghdadi declared a so-called caliphate in 2014 in large parts of Syria and Iraq that the extremists controlled. But ISIS was defeated in Iraq in 2017 and in March the SDF captured the last sliver of land that was held by the fighters in Syria.

The SDF has handed over hundreds of ISIS fighters to Iraqi authorities over the past two years, including Europeans, while some were repatriated to their home countries.

Earlier this year, Iraq tried 12 French ISIS fighters whom the SDF handed over to Baghdad in January, sentencing most of them to death.

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France at the time said the Iraqi court has jurisdiction to rule in the cases, though a spokeswoman reiterated the French government's opposition to the death penalty.

President Trump and other U.S. officials have repeatedly pressed other nations across Europe and the Middle East to take back the detainees from their countries. But international leaders have been largely reluctant and have been slow to take any back.

The Associated Press contributed to this report.