Former U.S. Special Representative to Ukraine for Negotiations Kurt Volker, when testifying in Congress in connection with the impeachment inquiry of U.S. President Donald Trump, said that Trump's telephone conversation with Ukrainian President Volodymyr Zelensky on July 25 made a discouraging impression, CNN has reported.

Volker said after reading the memorandum of the conversation between the two presidents, he was surprised because he did not expect that the topic of the Biden family and the investigation of their activities would arise in the conversation.

"When you ask the President of Ukraine to cooperate with the U.S. Attorney General on this matter and study all this, how you see and how it happened, it has an explosive effect in our domestic policy," Volker said, according to transcripts of the deposition reported by CNN.

Volker said such a request from Trump could have an impact on other aspects of U.S.-Ukraine relations.

"This again creates problems with everything we try to do to develop bilateral relations, to strengthen our support for Ukraine, to strengthen our positions against Russia. And now all this has stopped… with internal political struggles in the United States, with internal political events casting a shadow over (relations with Ukraine). And I think this is extremely unfortunate for our policy towards Ukraine," Volker said.

On Monday, three House of Representatives committees published transcripts of the depositions former U.S. State Department adviser Michael McKinley, who resigned in October, and former U.S. ambassador to Ukraine Marie Yovanovitch. The depositions took place behind closed doors.

On Tuesday, the committees also published transcripts of the depositions of U.S. Ambassador to the EU Gordon Sondland and Volker.