New York City Mayor Eric Adams has been indicted on criminal charges in federal court in Manhattan, sources familiar with the matter told CNBC on Wednesday night.

The indictment, which remains sealed for now, at least in part accuses Adams of criminal conduct related to donations to his 2021 mayoral campaign.

A source said that the indictment mentions up to $20 million in donations that Adams' campaign received as a result of matching fund program that the city's Campaign Finance Program offers candidates for small-dollar donations made to them by residents of New York City. That program gives candidates up to eight times the amount of a small-dollar donation from an individual.

Adams, a former police captain, is at least the second New York mayor to be criminally charged while still in office, and is the first official in his administration to be charged as a result of multiple pending investigations that have ensnared the New York Police Department and the city's top schools' official.

It was previously known the Manhattan U.S. Attorney's Office was investigating Adams for potentially conspiring with the government of Turkey to funnel illegal donations into that campaign.

The New York Times on Monday reported that prosecutors had submitted grand jury subpoenas to City Hall, Adams, and his campaign in July demanding information related to five other countries: Israel, China, Qatar, South Korea, and Uzbekistan.

Adams issued a defiant statement Wednesday night after news of the indictment broke.

"I always knew that If I stood my ground for New Yorkers that I would be a target — and a target I became," he said.

"If I am charged, I am innocent and I will fight this with every ounce of my strength and spirit," said Adams, who after working in the Police Department served as a state senator and Brooklyn Borough president.

In a video statement later, Adams said, "I will request an immediate trial, so New Yorkers will know the truth."

The U.S. Attorney's Office and the FBI declined to comment on the indictment.

Earlier Wednesday, Rep. Alexandria Ocasio-Cortez, D-N.Y., called on the mayor to resign, saying the "flood of resignations and vacancies" resulting from various federal probes of administration officials "are threatening government functions."

"For the good of the city he should resign," Ocasio-Cortez said.

If Adams resigns before his first term in office ends, he would be replaced by New York City Public Advocate Jumaane Williams as acting mayor.