The official said the IS militants were handed over to the Iraqi side late Wednesday, and that they were now in a "safe place" under investigation.
The transfer comes as the U.S.-backed Syrian Democratic Forces is involved in a standoff over the final sliver of land held by IS in eastern Syria, close to the Iraqi border.
Many believe the IS threat won't end with the pocket's recapture and that an insurgency is underway. In a foreboding sign Thursday, the IS claimed responsibility for back-to-back suicide attacks that hit a village miles away, leaving more than a dozen people dead in a rare targeting of civilians.
A few hundred people — many of them women and terrified-looking children — were evacuated Wednesday from the group's tiny tent camp on the banks of the Euphrates River, signaling an imminent end to the territorial rule of the militants' self-declared "caliphate" that once stretched across a third of both Syria and Iraq.
Some 300 IS militants, along with hundreds of civilians believed to be mostly their families, have been under siege for more than a week in the tent camp in the village of Baghouz. It wasn't clear how many civilians remain holed up inside, along with the militants.
More trucks were sent in Thursday to the tip of a corridor leading to the camp to evacuate more people, but Associated Press journalists on the ground outside Baghouz said no civilians emerged.
"We thought more civilians will come out today and we sent 50 trucks over," said an SDF commander who goes by his nom de guerre, Aram. "We don't know why they are not coming out."
Another SDF official said IS militants closed the roads so the civilians could not come out, citing an absence of military pressure on the group as a possible reason.
Nearly 20,000 people left through a humanitarian corridor on foot from the IS holdout earlier this month but the militants closed the passage and no civilians left for a week until Wednesday. The presence of civilians and possibly senior members of the militant group in Baghouz have slowed the group's defeat.
Hundreds of women and children from Wednesday's evacuation could be seen in the middle of the desert on the way out of Baghouz, in what appeared to be a screening area in an open field. SDF fighters could be seen among them but journalists were not allowed to approach or film them. A large convoy of coalition vehicles, armored and vans, headed in their direction.
The U.S.-led coalition declined to comment on the evacuation.
The issue of captured foreign fighters in Syria poses a major conundrum for European and other countries whose nationals have been imprisoned in Syria. The SDF is holding more than 900 foreign fighters in prisons it runs in the country's north, many of them Iraqis and Europeans.
The Kurdish-led SDF — and more recently President Donald Trump — have called on these countries to take back their nationals. The SDF says it cannot afford to keep the captured foreigners in Syria, but few of their countries want them back.
Earlier this month, Iraqi Prime Minister Abdul-Mahdi said Iraq will take back all Iraqi IS militants in Syria, as well as thousands of their family members.
The Iraqi security official, who spoke Thursday on condition of anonymity in line with regulations, said the SDF are holding more than 20,000 Iraqis suspected of IS membership as well as their families in prisons and camps in northern Syria.
An Iraqi intelligence official said among those were around 500 Iraqi IS fighters, adding that these will be transferred back home in batches. The intelligence official, who also spoke on condition of anonymity, said the first group of 150 was transferred to Iraqi authorities aboard 16 pickup trucks Wednesday night and that they have been moved to the capital Baghdad for interrogation.
An Alabama woman who joined IS in Syria also made headlines after the U.S. said Wednesday she won't be allowed to return with her toddler son because she is not an American citizen. Her lawyer is challenging that claim.
The 24-year-old Hoda Muthana has said she made a mistake and regrets aligning herself with IS. She is now in a refugee camp in Syria along with others who fled the militants.
On Thursday, back-to-back suicide car bombings in a market in Deir el-Zour province killed 14 civilians and an SDF fighter, a commander said.
IS claimed responsibility through its Amaq news agency and said the attack targeted an SDF vehicle.
Adnan Afrin, of the SDF, said the two blasts went off in the village of Shahil, about 10 kilometers (6 miles) from the al-Omar Oil Field base. He said two suicide bombers stopped their cars and detonated their explosives in the market.
The Syrian Observatory for Human Rights reported a car bomb that was detonated remotely as a convoy that includes workers and technicians who work at the oil field was passing. The Britain-based war monitor said 20 were killed and others wounded.
Also on Thursday, a French diplomatic official and an SDF official said they are trying to verify reports that Fabien Clain, a Frenchman who is one of Europe's most-wanted members of IS, was killed in an airstrike in Syria.
Clain is considered a key figure in the 2015 attacks in Paris, and it was his voice that claimed responsibility in the name of Islamic State for the deadly onslaught in his home country. Neither official would be named given the ongoing verifications in Baghouz, and a U.S. military spokesman said the coalition could not confirm the reports.
The Baghouz pocket's recapture by U.S.-backed Syrian fighters would spell the territorial defeat of IS and allow Trump to begin withdrawing American troops from northern Syria, as he has pledged to do, opening a new chapter in Syria's eight-year civil war.
Associated Press writer Zeina Karam in Beirut contributed reporting.