By Scott Malone and Nate Raymond

BOSTON (Reuters) - The U.S. Justice Department has threatened to sue Harvard University to force it to turn over documents as it investigates whether the Ivy League school's admission policies violate civil rights laws by discriminating against Asian-American applicants.

The Justice Department cited a 2015 lawsuit that charges Harvard's affirmative action policies discriminate against Asian-American applicants, in a letter setting a Dec. 1 deadline for Harvard to hand over documents on its admission policies.

The Justice Department is probing the Cambridge, Massachusetts-based school's compliance with Title VI of the 1964 Civil Rights Act, according to the letter, dated Friday and seen by Reuters. The measure prohibits institutions that receive federal funding from discriminating based on race, color or national origin.

The squabble follows reports that the Justice Department had begun an investigation into whether university affirmative action admission policies broadly discriminate against white applicants.

"The Department is left with no choice but to conclude that Harvard is out of compliance with its Title VI access obligations," the letter reads.

Attorneys for Harvard in an Oct. 6 letter to the Justice Department seen by Reuters questioned the timing of the probe.

"It is exceptionally unusual for the department to resurrect a complaint filed nearly 2-1/2 years earlier," the letter written by university attorney Seth Waxman read.

Harvard has long maintained that its admissions policies are fully compliant with U.S. laws and has worked to increase the amount of financial aid it offers to ensure economic, as well as racial, diversity in its classes.

The school said earlier this year that just over half of the freshmen admitted in 2017 were women, more than one in five was Asian and almost 15 percent African-American.

"The university will certainly comply with its obligations under Title VI," Harvard spokeswoman Anna Cowenhoven said in a statement on Tuesday. "We have an obligation to protect the confidentiality of student and applicant files and other highly sensitive records."

Affirmative action programs in higher education were meant to address racial discrimination. The Supreme Court has ruled universities may use affirmative action with the aim of helping minority applicants get into college.

U.S. conservatives have said that in helping black and Latino applicants, affirmative action can hurt white people and Asian-Americans.

"The Department of Justice takes seriously any potential violation of an individual's civil and constitutional rights,," Justice Department spokesman Devin O'Malley said in an e-mail.

(Reporting by Scott Malone and Nate Raymond; Editing by Susan Thomas and Andrew Hay)