Several days after Donald Trump's phone call with the leader of Ukraine, a top White House lawyer instructed a senior national security official not to discuss his grave concerns about the leaders' conversation with anyone outside the White House, according to three people familiar with the aide's testimony.

Lieutenant Colonel Alexander Vindman testified he received the instruction from John Eisenberg, the top legal adviser for the National Security Council, after White House lawyers learned on July 29 a CIA employee had anonymously raised concerns about the Trump phone call, the sources said.

The directive from Eisenberg adds to an expanding list of moves by senior White House officials to contain, if not conceal, possible evidence of Trump's attempt to pressure Ukrainian President Volodymyr Zelensky to provide information that could be damaging to former Vice-President Joe Biden.

The instruction came after White House officials had discussed moving a rough transcript of the call into a highly classified computer server. Eisenberg would later be involved in the Administration's battle to keep an explosive whistleblower complaint about the call from being shared with Congress.