HANOI (REUTERS) - United States President Donald Trump said on Thursday (Feb 28) that his former personal lawyer, Michael Cohen, had lied "a lot" in testimony to a congressional hearing in Washington.

However, he added that Cohen had told the truth when he said there had been no collusion with Russia.

Cohen testified on Wednesday, calling Mr Trump a "conman" who knew in advance about the release of stolen e-mails aimed at hurting his Democratic rival in the 2016 election campaign.

Mr Trump called the testimony "shameful".

"He lied a lot," Mr Trump told a news conference in Vietnam after a second summit with North Korean leader Kim Jong Un.

"He didn't lie about one thing: He said 'no collusion'."

Mr Trump criticised the timing of the hearing, saying it should not have been happened while he was on an important overseas trip, and it could easily have been postponed for a few days.

"Having a hearing like that, really was a terrible thing," he said.

Mr Trump has called an investigation into possible collusion between Moscow and his presidential campaign a "witch hunt".

Cohen, 52, was a close aide of Mr Trump for years and his testimony could increase the legal and political pressure on the Republican President.

But Cohen did not appear to disclose a "smoking gun" that could sink his former boss.

Cohen said he had no direct evidence that Mr Trump or his campaign colluded with Moscow during the election campaign.

Possible collusion is a key theme of Special Counsel Robert Mueller's Russia investigation, which has dogged the President during his two years in office. Mr Trump has repeatedly denied the allegation, as has the Kremlin.