"Your brothers the heroes of (the northern city of) Kirkuk were able to take hostage one of the most important collaborators of the rafida," the group said in an internet statement, using a pejorative term to refer to the Shia majority in Iraq.
"She is named Salma Jassem Hammadi and works as a translator in the large US base in Tikrit... and gathers information on the mujahedeen in Kirkuk," said the statement, the authenticity of which could not be independently verified.
The statement quoted the translator as saying Shia Grand Ayatollah Ali al-Sistani "issued a fatwa (to Shia ordering them) to work with US forces in order establish a Persian state and get rid of Sunnis."
The Islamist group said a "document was found in her possession confirming her statements" as well as "five medals offered by the US army for her services".
The statement said the translator would be "submitted to an investigation", and that "after finishing the inquiry and taking advantage of the information" gathered, it would broadcast a video of Hammadi and another woman translator it identified only as Mayyada.
It gave no further details on the second translator.
The group posted on its site photos of Hammadi apparently taken at the US base in Tikrit and a document it said was printed by "the Supreme Council for the Islamic Revolution".
Ansar al-Sunna is an Islamist group with links to the al-Qaeda network, and has claimed responsibility for executing hostages and carrying out numerous attacks in Iraq.