President Trump has dismissed concerns that Turkey's incursion into northern Syria could enable hundreds of hardened ISIS fighters to go free as a problem for other countries. There are more than 10,000 ISIS detainees held in jails across northern Syria run by America's long-time Kurdish allies, the Syrian Democratic Forces (SDF). The Kurds had warned that a Turkish offensive against them, which began on Wednesday, would force them to turn their attention from ISIS, to self defense.

About 2,500 of the ISIS detainees held in Syria are believed to be highly dangerous foreign fighters from Europe and elsewhere. On Wednesday, the U.S. military confirmed to CBS News that it had taken custody of two of those prisoners, a pair of British nationals suspected of involvement in the brutal murder of American hostages.

"Face the Nation" host Margaret Brennan said Thursday that the two Brits who had been held jointly by the U.S. and the Kurds were being moved to Iraq, where the U.S. has bases and can be more confident in their security.

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Speaking on Wednesday, Mr. Trump seemed unconcerned by warnings that thousands of other ISIS fighters — and thousands of their family members who are held in separate facilities in the region, including many who still sympathize with the extremist group — could escape amid mounting chaos.

"Well they're going to be escaping to Europe, that's where they want to go, they want to go back to their homes," Mr. Trump said at the White House.

The SDF, former U.S. military officials and serving American lawmakers — including one of Mr. Trump's staunchest supporters in Congress — have warned the president's decision to move American forces out of northern Syria to make way for the Turkish incursion targeting the Kurds could give ISIS room to regroup.

They have also chastised Mr. Trump for what they say amounts to an abandonment of a U.S. ally that lost 11,000 lives helping American forces push ISIS out of northern Syria.

"Pray for our Kurdish allies who have been shamelessly abandoned by the Trump administration. This move ensures the reemergence of ISIS," Republican Sen. Lindsey Graham, one of the president's most ardent defenders, tweeted Wednesday.

President Trump appeared on Wednesday to downplay the U.S. alliance with the Kurds, telling reporters that the ethnic group which has lived across a region spanning at least three countries for hundreds of years, "didn't help us in the Second World War, they didn't help us with (the D-Day landings in) Normandy."

Mr. Trump said he also wasn't worried about the perceived abandonment of an ally making it more difficult for the U.S. to form military partnerships in the future. "Alliances are easy," he said, adding: "With all of that being said, we like the Kurds."

"Stabbed in the back"

CBS News correspondent Charlie D'Agata reported Wednesday that SDF commanders feel "stabbed in the back" by Mr. Trump's decision to move the U.S. forces, who for years have protected the Kurds from any Turkish aggression just by being there.

Mr. Trump had repeatedly demanded that European countries, particularly France and Germany, take back their citizens who joined ISIS in Syria, but CBS News' Holly Williams recently visited one of the prisons — the first U.S. network correspondent to do so — and she found at least two detainees who said they were American citizens.

On Wednesday, Mr. Trump cited European nations' reluctance to take back their nationals from Syria, but he appeared to suggest the prisoners were in U.S. custody rather than held by America's now-distracted Kurdish allies.

"Europe didn't want them from us," the president said. "We could have given it to them, they could have trials, they could have done whatever they wanted. But as usual, it's not reciprocal."

Risky battle ramps up

D'Agata reported the first day of the Turkish offensive was more intense than expected. They unleashed an onslaught of artillery and airstrikes targeting the Syrian towns of Tal Abyad and Ras al-Ain, and the Turkish government confirmed Thursday that a ground offensive was also underway.

The attacks have left residents in Kurdish-controlled villages that were only recently liberated from ISIS with nowhere to hide, and once again running for their lives.

Turkey says the offensive is aimed at establishing a buffer zone along its southern border free of Kurdish militia members. They may be U.S. allies, but Ankara considers the SDF militia members terrorists linked to a separatist movement in the south of Turkey.

Turkey's President Recep Tayyip Erdogan said Thursday that the offensive had already killed 109 "terrorists," but an independent war monitoring group based in Britain said at least six civilians were killed in the first day of Turkey's cross-border operation, along with two two SDF civilian workers and 19 SDF fighters. They said 13 more civilians and 39 SDF fighters were wounded.

Commanders of the Kurdish-led SDF told CBS News they've had to put their operations against ISIS on hold to confront the Turkish invasion, and on Wednesday a senior U.S. military official confirmed to CBS News that operations against ISIS were "effectively paused."

The SDF said the Turkish offensive was jeopardizing security at the overcrowded prisons housing the thousands of ISIS inmates, risking breakouts and a possible ISIS resurgence.

Kurdish officials said Turkish shells hit one of the prisons on Wednesday night, but there was no immediate word on casualties, or any escapes, after that attack.

In a recently released audio recording, ISIS leader Abu Bakr al-Baghdadi called on members of the extremist group to do all they could to free detainees in jails and camps across northern Syria.

SDF spokesman Mustafa Bali said earlier this week that the U.S. troop redeployment in northern Syria would directly help ISIS, which still has many fighters in the region despite losing control of the territory it held.

Bali warned that "especially in the recently-liberated areas," ISIS would "seize the opportunity of such an (Turkish) invasion, and it may return to impose their control."

On Wednesday, CBS News analyst Admiral Sandy Winnefeld, a former Vice Chairman of the Joint Chiefs of Staff, said the Turkish offensive could have serious national security repercussions.

"If the Turkish incursion results in backing off pressure on ISIS in Syria and the release of hundreds of ISIS prisoners, that is potentially very destabilizing," Winnefeld said. "It poses a threat not only to the United States, but to a lot of our partners in Europe and elsewhere in the region."