John Bolton will be blocked from testifying at Donald Trump’s impeachment trial, the president has indicated, despite the former national security adviser insisting he would do so if he received a subpoena.

Trump claimed in an interview with Fox News host Laura Ingraham on Friday night he would “love everybody to testify”, including Bolton, the secretary of state, Mike Pompeo, and acting chief of staff, Mick Mulvaney.

But he went on to say “there are things that you can’t do from the standpoint of executive privilege”.

“Especially a national security adviser,” Trump added. “You can’t have him explaining all of your statements about national security concerning Russia, China and North Korea, everything. You just can’t do that.”

Asked if that meant he would invoke executive privilege to prevent Bolton from testifying, Trump said: “I think you have to for the sake of the office.”

Democrats believe Bolton, ousted by Trump last September, has key insight into the president’s failed efforts to secure a so-called quid pro quo with the government of Ukraine.

Trump is alleged to have demanded investigations into a conspiracy theory about election interference and unsubstantiated allegations of corruption involving Joe Biden, a possible rival in November’s election, in return for almost $400m in crucial military aid and a White House meeting.

Those efforts led to impeachment hearings in the Democrat-led House and ultimately two articles of impeachment, for abuse of power and obstruction of Congress, that the House speaker, Nancy Pelosi, said on Friday she would send to the Senate early next week.

Impeachment hearings contained dramatic testimony from White House aides, including that Bolton had referred to approaches to Ukraine as a “drug deal” and refused to get involved.

Bolton, who was ambassador to the United Nations under George W Bush, surprised the White House earlier this week by announcing he would testify at Trump’s Senate trial if subpoenaed to do so.

In a statement he said he had tried to “resolve serious competing issues” in weighing “my obligations both as a citizen and as a former national security adviser” and concluded that he was prepared to testify.

There remains significant doubt, however, that the issue will come to a head, irrespective of Trump’s pronouncement. The Senate majority leader, Mitch McConnell, has joined other leading Republicans in asserting that no witnesses should be called, and claimed on Thursday to have enough votes to start the trial on that basis.

Democrats need to persuade four Senate Republicans to join them in a vote on trial rules to allow witnesses. They received a glimmer of hope Friday when Susan Collins, a moderate from Maine, said she had been working with a small group of colleagues to find a compromise.

“We should be completely open to calling witnesses,” Collins said, according to the Bangor Daily News.

Trump’s removal is a long shot. A two-thirds majority of 100 senators is required for a conviction and there are few indications any Republicans are likely to break ranks.

Trump also had scathing words for Pelosi in the Fox News interview, saying it was “ridiculous” she had held on to the articles of impeachment for several weeks.