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.
"What I see right now troubles me. I see a state of conflict between the foreign policy professionals and someone who says he's acting on behalf of the president but frankly I don't know if that is the case," Rice, who also served as Bush's national security adviser, said at a conference in Abu Dhabi.
"It is troubling. It is deeply troubling," she said.
The House of Representatives' inquiry centers on Trump's July 25 request to Ukraine's president that he investigate former U.S. Vice President Joe Biden, a leading 2020 Democratic presidential candidate, and Biden's son Hunter, who was on the board of a Ukrainian gas company.
Witnesses in the inquiry have described how Giuliani and others worked to pressure Ukraine into announcing an investigation of the Bidens. They said Trump and his administration sought to tie $391 million in security aid to Ukraine to an investigation, as well as a White House meeting.
The United States has been highly supportive of Ukraine against the threat from Russia, especially after the takeover of the Crimea in 2014.
Rice, who went to the White House as a Soviet specialist, said any non-official persons carrying messages for the U.S. administration should make sure they are carrying the same message as the secretary of state or national security adviser.
"This is just not a good thing. The world shouldn't get confusing messages from the United States of America," she said.
Rice also criticized the July 25 call in which Trump made the request of Ukraine's newly elected president, Volodymyr Zelenskiy.
"The call is murky, it is really murky. I don't like for the president of the United States to mention an American citizen for investigation to a foreign leader. I think that is out of bounds," she said.