Rep. Adam Schiff, the chairman of the House Intelligence Committee, has asked Vice President Mike Pence to declassify the contents of his Sept. 18 call with Ukrainian President Volodymyr Zelensky.

Schiff sent a letter requesting the declassification to Pence today.

"While Ms. [Jennifer] Williams already testified generally to the phone call in question, declassification of this supplemental testimony will allow the Congress to see further corroborative evidence as it considers articles of impeachment, and provide the public further understanding of the events in question," a House Intelligence committee official told CNN.

About the call: Williams, an aide to Pence, told lawmakers she listened to a phone call between Pence and Zelensky, which took place on Sept. 18 after the hold on US security assistance to Ukraine was lifted, according to a transcript of her closed-door deposition.

Williams was asked if on the call Pence gave Zelensky any advice on how to approach Trump when they met at the forthcoming United Nations General Assembly meeting. Williams said Pence told Zelensky, "President Trump would be eager to hear about President Zelensky's progress in his reform agenda."