Cohen lying to reduce prison time: Trump

On the eve of former Trump lawyer Michael Cohen's testimony before the US House, Donald Trump has accused him of lying to get a reduced prison sentence.

Cohen has pleaded guilty to lying to US Congress about the project, which he says Trump knew about. (AAP)

US President Donald Trump is lashing out at his former lawyer and fixer, Michael Cohen, before Cohen testifies at a public hearing.

Trump is distancing himself from Cohen via a tweet from Hanoi, Vietnam, where he has travelled for a summit with North Korean leader Kim Jong-un.

Trump said Cohen was just "one of many lawyers who represented me (unfortunately)" and said "He is lying in order to reduce his prison time."

He also said Cohen "had other clients also" and "did bad things unrelated to Trump".

Cohen plans to tell a House committee on Wednesday that Trump knew ahead of time that WikiLeaks had emails damaging to 2016 Democratic rival Hillary Clinton's presidential campaign and that Trump is a "racist," a "con man" and a "cheat".

That's according to prepared testimony obtained by The Associated Press.

Cohen pleaded guilty to lying to Congress about a project in Russia.