Senior members of Congress on the House Oversight Committee say classified military documents show the Trump administration's former national security adviser, Michael Flynn, did not ask permission or inform the US government about payments he received for appearances before Russian organisations in 2015 and for lobbying that helped Turkey's government.
Flynn's failure to obtain permission from military authorities for the payments raises concern whether Flynn violated a constitutional ban on foreign payments to retired military officers. That's according to Republican Jason Chaffetz and Democratic Elijah Cummings on Tuesday.
The two leaders of the oversight committee said there was no evidence Flynn complied with federal law and that he should surrender the money paid and could be criminally prosecuted
The White House is refusing to provide lawmakers with information and documents related to Flynn's security clearance.
The White House was responding to requests made last month. The committee made six requests, and the White House cited reasons it could not comply with each of them.
Trump has said he fired Flynn because of misleading comments he made to the vice president about his discussions with the Russian ambassador during the transition.
Flynn is among the Trump associates being investigated by Congress and the FBI for possible ties between the Trump campaign and Russia.
Former acting attorney general Sally Yates is scheduled to appear at a congressional hearing next month on Russian interference in the 2016 US presidential election.
Yates is to appear May 8 along with James Clapper, the former director of national intelligence.
The hearing before a Senate Judiciary subcommittee was announced on Tuesday morning.
It will mark her first appearance on Capitol Hill since she was fired in late January after refusing to defend Trump's travel ban.