US President Donald Trump has said that the White House will probably release a transcript of a second call with Ukrainian President Volodymyr Zelenskiy on Tuesday.

"We have another transcript coming out which is very important," Mr Trump told reporters at Joint Base Andrews before leaving for a visit to Tuscaloosa, Alabama.

"I will give you a second transcript, because I had two calls with the president of Ukraine."

He added that this second transcript would probably be released on Tuesday.