Bolton would 'know nothing' of impeachment trial: Trump

US President Donald Trump claimed John Bolton would "know nothing" about the Senate impeachment trial, a day after the former national security advisor said he was ready to testify.

Trump has become the third president in US history to be impeached as the House of Representatives formally charged him with abuse of power and obstructing the Congress, setting up a Senate trial that will decide whether he remains in office after three tumultuous years.

"It'll be up to the Senate. He (Bolton) would know nothing about what we're talking about because the Ukraine president has said no pressure whatsoever," Trump told reporters in response to a question.

He addressing reporters at the Oval Office during a joint media appearance with visiting Greek Prime Minister Kyriakos Mitsotakis.

Will you be OK if John Bolton testifies? Trump was asked.

On Monday, Bolton had said that he had concluded that, if the Senate issues a subpoena for his testimony, he was prepared to testify.

Trump said, That's going to be up to the lawyers. It'll be up to the Senate, and we'll see how they feel.

He (Bolton) would know nothing about what we're talking about, because as you know the Ukrainian government came out with a very strong statement no pressure no anything and this from the boss," he said.

"That's from the president of Ukraine. The foreign minister came out with a statement that was equally as strong," Trump said.