The two members of the group still in the field, Alexanda Amon Kotey and El Shafee el-Sheikh, both from Britain, were captured in January in eastern Syria by the US-backed Syrian Democratic Forces.
"The two are believed to have acted as guards and interpreters involved in ISIS' illegal captivity of Western hostages, and are thought to have links to the British terrorist often called 'Jihadi John," the official said in a statement.
The US official did not give any information on the condition of the two or what would happen to them.
Of the other two members of the "Beatles" group, Mohammed Emwazi -- nicknamed "Jihadi John" -- was killed in 2015 in a drone strike by the US-led coalition fighting the Islamic State group.
"Jihadi John" was the leader of the group, known for using a knife to kill hostages in a string of videotaped beheadings, including of US journalists James Foley and Steven Sotloff.
The fourth member, Aine Davis, is being held in Turkey.
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