WASHINGTON—A top U.S. diplomat in Kyiv said President Trump made nearly $400 million in aid contingent on the Ukrainian president investigating Democratic presidential candidate Joe Biden and alleged Ukrainian interference in the 2016 U.S. election, in prepared testimony that shed new light on the central question facing the impeachment inquiry.

Bill Taylor, the acting U.S. ambassador to Ukraine, said Tuesday he grew concerned about dual channels through which the Trump administration was conducting foreign policy toward Kyiv—one...