A Pentagon inspector general is launching its own investigation into whether president Donald Trump's former national security adviser Michael Flynn took payments from foreign entities without approval, according to a letter released by House Democrats on Thursday.

The inspector general of the Department of Defence is probing whether Mr Flynn had "failed to obtain required approval prior to receiving any emolument from a foreign government," according to the April 11 letter to the House Oversight and Government Reform Committee.

Documents released by lawmakers on Thursday show Mr Flynn was warned when he retired from the military in 2014 not to take foreign money without "advance approval" by Pentagon authorities.