WASHINGTON (AP) — House Republicans are warning Hunter Biden that they will move to hold him in contempt of Congress if he doesn't appear this month for a closed-door deposition, raising the stakes in the growing standoff over testimony from President Joe Biden's son.

Hunter Biden has insisted that he will only testify to the House if it's in public. But in a letter sent to his attorney Wednesday, top Republicans told him that their subpoena for a closed-door deposition on Dec. 13 is non-negotiable.

"Contrary to the assertions in your letter, there is no 'choice' for Mr. Biden to make," wrote Rep. James Comer, R-Ky., the chairman of the House Oversight Committee, and Rep. Jim Jordan, R-Ohio, the chairman of the House Judiciary Committee.

Comer and Jordan added that seeking a private session before a public hearing is an approach both parties have historically taken when deposing witnesses.

Hunter Biden has told Republicans he will not testify behind closed doors because information from those interviews can be selectively leaked and used to "manipulate, even distort, the facts and misinform the American public."

The response to the committee was in line with the more forceful approach Hunter Biden's legal team has taken in recent months as congressional Republicans pursue an impeachment inquiry seeking to tie his father to his business dealings.

President Biden on Wednesday dismissed as "lies" claims that he behaved illegally or unethically regarding the business dealings of his son. The question was asked after the president gave remarks about funding the war effort in Ukraine and comes after polling by The Associated Press-NORC Center for Public Affairs that found most U.S. adults believe the president acted illegally or unethically regarding his son.

"I'm not going to comment on it," Biden said of the GOP allegations. "I did not. It's just a bunch of lies. They're lies."

Republicans have so far failed to uncover evidence directly implicating the president in any wrongdoing. But questions have arisen about the ethics surrounding the Biden family's international business, and lawmakers insist their evidence paints a troubling picture of "influence peddling" in the family's business dealings, particularly with clients overseas.

The early-November subpoenas to Hunter Biden and others were the inquiry's most aggressive steps yet, testing the reach of congressional oversight powers, and White House has questioned their legitimacy.

The criticism has prompted Republicans to plan a vote next week to formally authorize their impeachment inquiry in an effort to strengthen their legal standing if the subpoena battle drags into court.

"The House has no choice if it's going to follow its constitutional responsibility to formally adopt an impeachment inquiry on the floor so that when the subpoenas are challenged in court, we will be at the apex of our constitutional authority," House Speaker Mike Johnson told reporters.

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Associated Press writer Josh Boak contributed to this story.