NEW YORK (REUTERS) - Former US president Donald Trump on Monday (Dec 20) sued New York Attorney-General Letitia James, seeking to stop her civil fraud investigation into his company.

He filed the lawsuit in federal court in Syracuse, New York, after it was reported that Ms James' office would seek to question Mr Trump as it probes whether his company, the Trump Organisation, manipulated the valuations of its real estate properties.

Mr Trump and the company, which is also a plaintiff in the case, claim that Ms James has violated their rights under the US Constitution by pursuing a politically motivated investigation.

"By filing this lawsuit, we intend to not only hold her accountable for her blatant constitutional violations, but to stop her bitter crusade to punish her political opponent in its tracks," Mr Trump's attorney, Ms Alina Habba, said in an e-mailed statement.

A spokesman for Ms James' office did not immediately respond to a request for comment.

The civil probe is related to, but separate from, a more-than-three-year-old criminal probe by Manhattan District Attorney Cyrus Vance of the Trump Organisation's business practices, which Ms James joined in May. It includes a focus on whether the Trump Organisation overstated the value of some real estate assets to obtain loans and tax benefits.

In July, the company and long-time chief financial officer Allen Weisselberg pleaded not guilty to criminal charges in what a prosecutor in Mr Vance's office called a "sweeping and audacious" 15-year tax fraud.

In Monday's lawsuit, Mr Trump and the company claim that Ms James, a Democrat, is motivated by partisan bias against Mr Trump, a Republican, pointing to public statements she made against the former president before she was elected to her position. They are seeking a court order barring the investigation from going forward.