It was unclear how far the investigation has progressed, and there was no indication that prosecutors in Manhattan have decided to file additional charges in the case. A spokeswoman for the United States attorney in Manhattan, Geoffrey S. Berman, declined to comment.

Mr. Giuliani said that federal prosecutors had no grounds to charge him with foreign lobbying disclosure violations because he said he was acting on behalf of Mr. Trump, not the Ukrainian prosecutor, Yuriy Lutsenko, when he collected the information on Ms. Yovanovitch and the others and relayed it to the American government and the news media.

“Look, you can try to contort anything into anything, but if they have any degree of objectivity or fairness, it would be kind of ridiculous to say I was doing it on Lutsenko’s behalf when I was representing the president of the United States,” Mr. Giuliani said. Mr. Lutsenko had chafed at Ms. Yovanovitch’s anticorruption efforts and wanted her recalled from Kiev.

Mr. Giuliani also said he was unaware of any investigation into him, and he defended the pressure campaign on Ukrainians, which he led, as legal and above board.

CNN and other news organizations reported that federal prosecutors were scrutinizing Mr. Giuliani’s financial dealings with his associates, but it has not been previously reported that federal prosecutors in Manhattan are specifically investigating whether he violated foreign lobbying laws in his work in Ukraine.

Ms. Yovanovitch told impeachment investigators on Friday that Mr. Trump had pressed for her removal for months even though the State Department believed she had “done nothing wrong.”

Mr. Giuliani had receded from the spotlight in recent years while he built a brisk international consulting business, including work in Ukraine. But he re-emerged in the center of the political stage last year, when Mr. Trump retained him for the special counsel’s investigation into Russian election interference.