Condoleezza Rice, a secretary of state under Republican President George W. Bush, said on Monday reports of an unofficial U.S. policy being carried out in Ukraine were "deeply troubling."
Rice was critical of President Donald Trump during his 2016 presidential run but has been more restrained since the Republican took office.
State Department officials have testified in the Democratic-led impeachment inquiry about an "irregular channel" of people involved in Ukraine policy, including Rudy Giuliani, Trump's personal lawyer. Public hearings begin this week.