Robert De Niro has reignited his feud with Donald Trump, labelling the US president a "nightmare".

The revered actor, 75, appeared on long-running US comedy show Saturday Night Live last week, playing special counsel Robert Mueller in a sketch alongside Alec Baldwin as Mr Trump.

Mr. Mueller is in charge of the investigation into Russia's alleged interference in the 2016 presidential campaign. The president has dismissed the probe as a "witch hunt".

De Niro, star of films including The Godfather Part II, Goodfellas, Meet The Parents, and Casino, appeared on CNN on Monday to discuss his animosity towards the president.

"I never thought in my lifetime, and I have seen a lot in my lifetime, I never thought - I was working out with my trainer the other day, we have seen horrible things over the years, but this is one of the worst I have ever seen and ever thought I would see, but it's real," he said.

During the Tony Awards in June, De Niro took to the stage and said, "f... Trump". The president responded by saying the Raging Bull star was "punch drunk and "a very Low IQ individual".

On CNN on Monday, De Niro was asked why he was so vocal about the president.

He replied: "I know what kind of person this guy is, he is a New Yorker who I never would want to meet, and now he's president. And the reason I wouldn't want to meet him is because of the kind of person he is."

"As we all know now, there's nothing new. It's disgraceful but we'll get past it. It will be like a nightmare that you remember."

In October, De Niro was one of the high-profile opponents of the president who had suspicious packages addressed to them. Police were called to De Niro's Tribeca Productions in Manhattan.

The discovery came after a series of glass-filled pipe bombs targeting Hillary Clinton, former president Barack Obama, other prominent Democrats and CNN were thwarted without injury.