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WASHINGTON — Ronny Jackson’s confirmation hearing to lead the Department of Veterans Affairs, scheduled for Wednesday afternoon, has been postponed because of unspecified concerns about Jackson's background, two sources told NBC News.

A White House official said the scope of the allegations are not yet clear. An aide to a Republican senator on the committee said members are continuing to dig into Jackson's recent history.

The sources said senators on the Veterans Affairs Committee became aware of these concerns only recently — within the past several days — and raised them with the White House.

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The Washington Post first reported the postponement.

Veterans Affairs Committee Chairman Johnny Isakson, R-Ga., and ranking member Sen. Jon Tester, D-Mont., said Tuesday that the timing for the hearing was up in the air.

"The Senate Committee on Veterans' Affairs is postponing the hearing to consider the nominee to be secretary of the U.S. Department of Veterans Affairs in light of new information presented to the committee," they said in a statement. "We will continue looking into these serious allegations and have requested additional information from the White House to enable the committee to conduct a full review."

Jackson told NBC News on Tuesday that he was looking forward to the hearing and declined to comment on the allegations against him. "I'm looking forward to getting (the hearing) rescheduled and answering all the questions," he said on his way into a meeting with a lawmaker on Capitol Hill.

Jackson is the White House physician — a position he also held under former President Barack Obama. President Donald Trump announced his choice last month, after firing the department's former secretary, David Shulkin, and after promoting Jackson to rear admiral.

In January, Jackson briefed the media on Trump's health, telling reporters the president has "a lot of energy and a lot of stamina" and was in "excellent" health.

But Jackson has no experience running a large bureaucracy — an issue raised by Democrats and Republicans alike.

House Minority Leader Nancy Pelosi, D-Calif., called his "lack of VA leadership" experience "concerning," while former Republican National Committee Chairman Michael Steele put it this way on MSNBC:

"Unless he was a mayor of a town somewhere in America we don’t know about, he won’t have a clue what to do."

Jackson and Haake reported from Washington, D.C., and Stelloh reported from San Francisco, California