A top White House official with the National Security Council told lawmakers Tuesday that U.S. President Donald Trump’s July 25 telephone call with his Ukrainian counterpart was “improper” and “inappropriate.”

“It is improper for the president of the United States to demand a foreign government investigate a U.S. citizen and political opponent,” Lt. Col. Alexander Vindman, the National Security Council’s director of European affairs, told the House of Representatives Intelligence Committee.

During the call Trump repeatedly pressed Ukrainian President Volodymyr Zelensky to open a corruption probe into Democratic front-runner Joe Biden and his son, Hunter, asking for a “favor.”

Read more: White House aide says Trump's Ukraine call 'improper'