(CNN) Tim Morrison, a top Russia and Europe adviser on President Donald Trump's National Security Council, is expected to testify before House impeachment investigators next week and corroborate key elements of a top US diplomat's account that Trump was pressing for Ukraine to publicly announce investigations into the Bidens before he would greenlight US security assistance, according to sources.

Bill Taylor, the top US diplomat in Ukraine, said in extraordinary testimony on Tuesday that Trump pushed for Ukraine to publicly announce investigations, including one into former Vice President Joe Biden and his son, using as leverage the military aid the country sought to fight back against Russian aggression.

Morrison's testimony is expected to be significant because he is a current White House official whose name was cited 15 times in Taylor's opening statement, which Democrats view as damning for Trump.

Morrison also listened to the July 25 call between Trump and the Ukrainian leader, CNN reported earlier this month. His testimony would be one of the first from someone who heard the call directly. A transcript of the call was released by the White House but was not a full verbatim.

But two sources also tell CNN that Morrison will contend that he didn't see anything wrong with what the Trump administration did, while one of the sources said there will be "nuance" over what Morrison intends to say.

Read More