The US government said on Wednesday it had charged a Defense Department linguist with transmitting classified intelligence to a Lebanese national linked to Hezbollah, saying she revealed the names of key American assets and put their lives at risk.

Mariam Taha Thompson, 61, formerly of Rochester, Minnesota, was arrested by FBI agents at an overseas military base in Erbil, Iraq, on February 27. She was due to make an initial appearance on Wednesday in US District Court for the District of Columbia.

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"While in a war zone, the defendant allegedly gave sensitive national defense information, including the names of individuals helping the United States, to a Lebanese national located overseas," John Demers, the Justice Department's assistant attorney general for national security, said in a statement.

"If true, this conduct is a disgrace, especially for someone serving as a contractor with the United States military. This betrayal of country and colleagues will be punished."

The Justice Department said that during its investigation, it discovered audit logs showing a "notable shift" in Thompson's network activity on the Defense Department's classified systems.

The discovery came on December 30, 2019, a day after the US launched air raids against Iranian-backed forces in Iraq and the same day that protesters there stormed the US embassy in response.

The information Thompson was looking up involved dozens of files on human intelligence sources she had no need to access, including their names, photographs and other personally identifiable information, the department said.

In February, FBI agents searched her living quarters on the base and discovered a note, handwritten in Arabic, under her mattress, according to a sworn statement from an FBI agent contained in a court filing.

The note had the names of US intelligence informants. The note said these sources were collecting information for the US, and suggested that their phones be monitored and their target should be warned, according to the sworn statement.

In subsequent interviews with the FBI, Thompson waived her Miranda rights and told the agents she had passed along the information from the note to a Lebanese foreign national, according to the statement.

Court documents identify the person only as a co-conspirator in whom Thompson had a romantic interest. She told the FBI she did not know he was connected with Hezbollah, which was first designated as a terrorist group by the US in 1997, according to the court filing.

To share the intelligence, Thompson claimed she had memorised the classified information, wrote it down and conveyed it to the co-conspirator through a secure messaging application on her phone, according to the filing. She apparently later amended her statement, saying she was 70 percent sure she had not transmitted the note.

However, investigators said in the court filing that they had developed solid evidence to suggest Thompson was sending details to the individual about the human intelligence sources, including information she wrote on a second Arabic note.