By Patricia Zengerle

WASHINGTON (Reuters) - The U.S. Justice Department is preparing criminal corruption charges against New Jersey Democratic Senator Robert Menendez, CNN reported on Friday.

Citing unnamed sources briefed on the case, CNN said the charges center on allegations Menendez used his office to promote the business interests of a Democratic Party donor and friend in exchange for gifts.

Menendez's office said he would speak to the media at 7 p.m. on Friday in Newark, New Jersey.

"As we have said before, we believe all of the senator's actions have been appropriate and lawful, and the facts will ultimately confirm that," Tricia Enright, Menendez's communications director, said in a statement.

Noting the official investigation of the matter was ongoing, she said the senator's office "cannot address allegations being made anonymously."

CNN said Attorney General Eric Holder has given the green light for prosecutors to proceed with charges and an announcement could arrive in coming weeks.

Holder, who was traveling in South Carolina with President Barack Obama, said he could not comment.

Justice Department officials declined to comment.

Two law enforcement officials told Reuters the Federal Bureau of Investigation had been conducting a major corruption investigation of Menendez for some time, and one said it was nearing completion.

Menendez is the former chairman of the Senate Foreign Relations Committee, where he is the top Democrat. He has been a critic of the Obama administration's Cuba and Iran policies, but he is a staunch supporter of the White House on other issues.

Federal authorities have probed Menendez's relationship with Florida ophthalmologist Salomon Melgen, a Democratic donor who has been accused of over billing the Medicare program.

Media reports over the past two years alleging wrongdoing by Menendez have not been substantiated and at least one, involving underage prostitutes, has been discredited.

Menendez, who is Cuban-American, is among the most senior Hispanic politicians in the country. He was re-elected to a second term in the Senate in 2012 and spent 14 years in the U.S. House of Representatives.

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CNN said the government's case focuses on Melgen and, in part, on plane trips that Menendez took in 2010 to the Dominican Republic as a guest of Melgen.

In 2013, Menendez's campaign repaid Melgen $11,250 for a flight on Melgen's private plane three years earlier.

Prosecutors also are focusing on whether Menendez promoted Melgen's business interest in a Dominican Republic government contract for port screening equipment, CNN said.

(Additional reporting by Mark Hosenball and Elvina Nawaguna; Editing by Kevin Drawbaugh, Sandra Maler, Richard Chang, Leslie Adler and Andre Grenon)