Hillary Clinton has denounced Donald Trump’s bellicose language toward North Korea, believing his verbal aggression has rattled American allies and will set off a nuclear arms race in the region.

“We will now have an arms race – a nuclear arms race in East Asia,” Clinton said in an interview with CNN due to be broadcast on Sunday, in which she also criticised Trump’s threat to pull out of the international nuclear deal with Iran. “We will have the Japanese, who understandably are worried with missiles flying over them as the North Koreans have done, that they can’t count on America.”

Clinton, who was secretary of state under Barack Obama from 2009 to 2013, stressed that she preferred a diplomatic solution; suggested Trump’s inflammatory rhetoric played into Kim Jong-un’s hands; and bemoaned Trump’s public undercutting of his secretary of state, Rex Tillerson, regarding his attempts to work with China and establish talks with Pyongyang.

“Diplomacy, preventing war, creating some deterrents is slow, hard-going, difficult work,” said Clinton, who declined to answer when asked if Tillerson should resign. “And you can’t have impulsive people or ideological people who basically say, ‘Well, we’re done with you.”’

On Friday Bob Corker, the Tennessee Republican who chairs the Senate foreign relations committee, continued his war of words with the president when he told the Washington Post Trump had “castrated” Tillerson.

“The greatest diplomatic activities we have are with China, and the most important, and they have come a long, long way,” Corker added. “Some of the things we are talking about are phenomenal. When you jack the legs out from under your chief diplomat, you cause all that to fall apart.”

Clinton recently released a book that recounts her 2016 election defeat by Trump – in which she won nearly 3m more ballots but lost in the electoral college. The White House did not immediately return a request to respond to her comments.

Clinton said Trump’s threat to pull out of the Iran nuclear deal was “dangerous” and was undermining the validity of US promises to other nations. On Friday, Trump accused Iran of violating the landmark 2015 accord, blaming Tehran for a litany of sinister behavior and hitting its main military wing, the Revolutionary Guard, with anti-terror penalties.

Trump did however break his own campaign pledge to rip up the Iran agreement, deciding not to pull the US out or re-impose nuclear sanctions, though he left that option on the table.

In the interview, which was recorded on Wednesday, Clinton said Trump’s insistence on decertifying the deal even though evidence has pointed to Iranian compliance “makes us look foolish and small and plays right into Iranian hands”.

“That is bad not just on the merits for this particular situation, but it sends a message across the globe that America’s word is not good,” she said. “We have different presidents, and this particular president is, I think, upending the kind of trust and credibility of the United States’ position and negotiation that is imperative to maintain.”

Clinton, who as secretary of state began negotiations on the deal, did say Iran was engaging in other dangerous behavior.