James Clapper, the former director of national intelligence, has said Donald Trump’s implication that officials involved in the whistleblower complaint should be treated as spies is “witness retaliation”, as more than 300 national security officials signed a letter saying the accusations against Trump amount to an “unconscionable abuse of power”.

As pressure continued to mount on Trump, the former secretary of defense and CIA director Leon Panetta also suggested the White House’s attempts to conceal Trump’s conversation with his Ukrainian counterpart suggested the administration was “at least thinking of a cover-up”.

Trump was covertly recorded telling US diplomats that the whistleblower’s source was “close to a spy” on Thursday. The president added: “You know what we used to do in the old days when we were smart with spies and treason, right? We used to handle it a little differently than we do now.”

Speaking to CNN, Clapper said of Trump’s statement: “At a minimum, it is witness retaliation.

“It actually conflicts with the spirit of the Intelligence Community Whistleblower Protection Act, because one of the purposes is to insulate complainants from retaliation,” Clapper said. “Clearly, this is retaliation threatened in the worst way.”

The whistleblower alleged that the White House placed the transcript of Trump’s call with Ukraine’s president, Volodymyr Zelenskiy, in a secure, limited access, server.

“I never experienced the step of isolating the president’s calls,” Panetta told USA Today.

“They were always kept in the same place. This is clearly an indication that they were at least thinking of a cover-up if not, in fact, doing that. It’s a very serious matter because this is evidence of wrongdoing.”

As the House speaker, Nancy Pelosi, ordered Democrats to narrow the focus of the impeachment inquiry to the Trump-Ukraine scandal, more than 300 former national security officials said in a joint statement that the accusations against Trump amount to an “unconscionable abuse of power”.

“President Trump appears to have leveraged the authority and resources of the highest office in the land to invite additional foreign interference into our democratic processes. That would constitute an unconscionable abuse of power,” they wrote.

“It also would represent an effort to subordinate America’s national interests – and those of our closest allies and partners – to the President’s personal political interest.”

The signatories include William Burns, former deputy secretary of state, and Matthew Olsen, former director of the National Counterterrorism Center.