John Bolton is seen in August 2019 in Minsk, Belarus. Sergei Gapon/AFP/Getty Images

Top US diplomat to Ukraine Bill Taylor told Congress that former national security adviser John Bolton had expressed concern about a call between President Trump and Ukrainian President Volodymyr Zelensky, according to Taylor's opening statement.

“Ambassador Bolton opposed a call between President Zelensky and President Trump out of concern that it would ‘be a disaster,'" Taylor wrote.

Taylor said in August and September, he "became increasingly concerned" about the US' relationship with Ukraine. Taylor wrote that he was concerned that "our relationship with Ukraine was being fundamentally undermined by an irregular, informal channel of US policy-making and by the withholding of vital security assistance for domestic political reasons."

According to Taylor, Bolton recommended he "send a first-person cable to Secretary Pompeo directly, relaying my concerns."

"I wrote and transmitted such a cable on August 29, describing the ‘folly’ I saw in withholding military aid to Ukraine at a time when hostilities were still active in the east and when Russia was watching closely to gauge the level of American support for the Ukrainian government," Taylor said. "I told the secretary that I could not and would not defend such a policy.”