Kittleson was freed on Tuesday afternoon, said the official, who spoke to the Associated Press on condition of anonymity because he was not authorised to comment publicly.
He did not share her current whereabouts but said that prior to her release, she had been held in Baghdad.
The powerful Iran-backed Iraqi militia Kataib Hezbollah said in a statement earlier in the day it had decided to free Kittleson, who was abducted on March 31.
The group said its decision came "in appreciation of the patriotic stances of the outgoing prime minister" Mohammed Shia al-Sudani, without giving more details.
It added that "this initiative will not be repeated in the future".
The statement added that Kittleson must "leave the country immediately" upon her release.
Kataib Hezbollah had not previously acknowledged that it was the one responsible for Kittleson's abduction, although both US and Iraqi officials had pointed fingers at the group.
Two officials within the militia, who spoke on condition of anonymity, told the AP that in exchange for freeing Kittleson, several members of the group who had previously been detained by Iraqi authorities would be released.
Kittleson, 49, a freelance journalist, had lived abroad for years before the kidnapping, using Rome as her base for a time and building a journalism career across the Middle East, particularly in Iraq and Syria.
Like many freelancers, she often worked on a shoestring budget and without the protections afforded by large news organisations to staff.
She had entered Iraq again shortly before her abduction.
US officials have said that they warned her multiple times of threats against her but that she did not want to leave.
Iraqi officials have said that two cars were involved in the kidnapping, one of which crashed while being pursued near the town of al-Haswa in Babil province, southwest of Baghdad.
The journalist was then transferred to a second car that fled the scene.
Three Iraqi officials said earlier on Tuesday that attempts to negotiate her release had run into obstacles.
The two Iraqi security officials and one official from the Iran-aligned Coordination Framework political bloc spoke to the AP on condition of anonymity because they were not authorised to speak about the sensitive case publicly.
One of the security officials said that an official with the Popular Mobilisation Forces, a coalition of Iran-backed militias that is nominally under the control of the Iraqi military, had been tasked with communicating with the abductors to secure Kittleson's release but had run into difficulties in communicating with the Kataib Hezbollah leadership.
"The primary challenge is that the leaders of the Kataib militia - specifically, the commanders of the battalions - are nowhere to be found. No one knows their whereabouts, and the process of establishing contact with them is extremely complex," they said.
"These leaders have gone underground, maintaining no active lines of communication, out of fear of being targeted."
The political official said a message had been sent to the Kataib leadership to determine their demands in exchange for releasing the kidnapped journalist.
Iraqi authorities were willing to release six Kataib Hezbollah members who are currently detained, most of them in connection with attacks on a US base in Syria, they said.
The second security official said that to further complicate matters, the Iraqi official in charge of the case had not yet received the go-ahead from US officials to proceed with negotiations.
US officials did not immediately respond to requests for comment.
The US State Department previously said that it is working with the FBI to secure Kittleson's release.
Journalist advocacy groups had urged the US government to formally designate Kittleson a hostage, or "wrongful detainee," a designation that triggers an elevated level of response.
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