Former Democratic U.S. President Jimmy Carter on Tuesday called on President Donald Trump to cooperate with Congress' impeachment inquiry, saying his refusal to comply with lawmakers' requests has left Americans grappling for answers.

The Republican leader faces allegations he abused his office by pushing Ukraine to investigate his political rival Joe Biden. Earlier on Tuesday, the Trump administration blocked a key witness from appearing before three House committees, prompting a subpoena.

"Tell the truth, I think, for a change," Carter told MSNBC when asked what advice he would give Trump.