The Pentagon inspector general is investigating whether Michael Flynn, President Donald Trump's former national security adviser, accepted money from foreign entities without the required approval.
The new probe compounds the legal problems Flynn faces for taking hundreds of thousands of dollars from the state-run Russia Today (RT) television network and a firm owned by a Turkish businessman after he'd retired as an Army lieutenant general in 2014.
Senior lawmakers claimed this week that Flynn broke the law by failing to request and receive permission to accept $US45,000 to speak at a 2015 RT gala dinner at which he sat with Russian President Vladimir Putin.
If substantiated, such a failure would violate regulations rooted in the Constitution that bar current and retired military officers from accepting "emoluments" from foreign powers, the letter said.
White House spokesman Sean Spicer told reporters on Thursday that the inspector general's probe was "appropriate."
"If they think there's wrongdoing, then the department's inspector general should look into that," he said.
Spicer said Flynn did not undergo a security review before he was named Trump's first national security adviser.
"Why would you rerun a background check on someone who was the head of the Defence Intelligence Agency that had and did maintain a high-level security clearance?" he said.
However, a former senior White House official familiar with ethics and security clearance issues said that during the administration of former President Barack Obama, the White House Counsel's office always conducted its own background investigations of candidates for senior White House jobs, even if they already held security clearances.
These reviews included questionnaires and interviews, and would have included issues such as whether the candidate had drug problems, had hired prostitutes, or had personal or financial problems, the former official said.
Flynn is also the subject of congressional probes into Russian interference in the 2016 presidential election that US intelligence agencies have said was intended to sway the vote in favour of Trump over Hillary Clinton.
Russia denies the allegation.
Representative Elijah Cummings, the House committee's top Democrat, released other Defence Department documents showing that the DIA found no evidence that Flynn requested permission to accept foreign funds, despite a 2014 warning against taking such payments.
One document, a redacted version of a letter the DIA sent to the committee on April 7, said the agency had not found "any records referring or relating to LTG (Lieutenant General) Flynn's receipt of money from a foreign source."
Flynn's lawyer, Robert Kelner, said in a statement that Cummings mischaracterised the DIA letter, saying a partially redacted section confirmed that Flynn gave the Pentagon "information and documents on a thumb drive" about the RT event.
