DAVOS (Switzerland) • United States President Donald Trump yesterday said some potential witnesses in his impeachment trial would raise "national security" concerns, but the Senate will decide whether any of them will testify.

Mr Trump would "love" for his acting chief of staff, Mr Mick Mulvaney, and Secretary of State Michael Pompeo to testify, he told a news conference.

But he said Mr Mulvaney has already "expressed himself very well" in a Fox News interview and that Mr Pompeo presents "a national security problem".

"He knows some of my thoughts" on foreign leaders, Mr Trump said, expressing concern his views might become public in the trial.

Mr Trump has previously vacillated on whether witnesses should testify in his trial, expressing both expectations that people like Mr Pompeo and Mr Mulvaney would buttress his defence, and also concern about setting precedent for future presidents.

Mr Trump also said that he does not favour his former national security adviser John Bolton testifying. "He knows other things and I don't know if we left on the best of terms. I would say probably not. So you don't like people testifying when they don't leave on good terms," Mr Trump said.

He also said he would want Mr Rick Perry, adding that the former US energy secretary had asked him if he could testify. "I would rather go the long way," he said.

"You could call it presidential prerogative," Mr Trump said. "The way I look at it, I call it national security, for national security reasons. Executive privilege."

The Senate will ultimately decide whether to hear from witnesses, he said. "I'm going to head back and I'll be watching it," he said of the trial. "But it's really going to be up to the Senate."

Mr Trump also said he would love to be attending his impeachment trial, but that his lawyers had advised him against it.

"I'd love to go. I'd sort of love (to) sit in the front row and stare in their corrupt faces," Mr Trump told reporters, referring to Democratic senators as he wrapped up his trip to Davos.

But he added: "I think they (his lawyers) might have a problem."

Senators must decide whether to remove Mr Trump from office after he was impeached last month by the Democratic-controlled House of Representatives, on charges of abuse of power and obstruction of Congress, for pressuring Ukraine to investigate former Democratic vice-president Joe Biden, a political rival, and impeding the inquiry into the matter.

With experts opining that the impeachment trial could have a bearing on the presidential election later this year, Mr Trump announced that he was preparing to extend controversial restrictions on travellers from certain countries.

Mr Trump said his administration was preparing to add a "couple of countries" to the list of states whose citizens are subject to travel bans or severe restrictions on entry to the US. He said that the names of the new countries would be announced "very shortly".