WASHINGTON — Retired Navy Adm. Mike Mullen, the former chairman of the Joint Chiefs of Staff, said he is "extremely concerned" about heightened tensions between North Korea and the United States and expressed concern that disproportional responses and unintended consequences from any military strike could "really get out of control fast."

"I don't know where this goes in terms of a peaceful resolution," Mullen said on NBC's "Meet The Press" this weekend. "It's an incredibly difficult, complex problem, and we have rhetoric, some very strong rhetoric, coming from both North Korea, as well as from the United States. And that rhetoric, it seems to me, has taken away options, or it's reduced maneuver space, if you will, for leaders to make decisions."

A war of words escalated between the two countries last week after reports revealed an intelligence assessment that North Korea had gained the capability to fit a nuclear weapon on a missile, a move that signaled a major advancement in its military weapons program.

President Donald Trump then proclaimed that North Korea "will be met with fire, fury and, frankly, power the likes of which this world has never seen before" if it threatened the United States, and North Korea subsequently threatened to take aim at the U.S. territory of Guam.

Trump didn't back down from his words on Thursday, saying he may not have gone far enough, and he said Friday that North Korean leader Kim Jong Un "will truly regret it, and he will regret it fast," if any U.S. territory or ally were attacked.

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Mullen said language like that from the president "eliminates maneuver space for him."

"It looks like brinkmanship to me," he said. "And it looks like clearly he is at least verbally focused very specifically on the military options with the rhetoric that's out there. It's almost a fire and brimstone 'don't make another move or else.'"

Trump's national security adviser, H.R. McMaster, said on Sunday, however, that "the president doesn't draw red lines."

"What he does is he asks us to make sure that we have viable options for him," McMaster said on "Meet The Press." "Options that combine diplomatic, economic and military capabilities. And so that's what we've done."

Mullen repeated that the situation should be resolved peacefully.

"I think it's got to be resolved politically, diplomatically, through negotiations to ensure that we don't have a military conflict that could just get out of control," he said.

Mullen also said it's "guaranteed" that if Kim ever made the decision to use nuclear weapons, it would basically be "an act of suicide from him, because we would eliminate him and his regime immediately."

Mullen broke with former National Intelligence Director James Clapper, who has said the world will have to accept that North Korea will be a nuclear state.

"I don't accept that yet," Mullen said. "I recognize that as an option or an outcome, and certainly one option is to accept that and then contain him. Obviously, the concern you would have with that is somehow he has this weapon. And he's still somewhat of an unknown to us."

Mullen served under both Presidents George W. Bush and Barack Obama, leading the nation's armed forces from 2007 through 2011.

"There are hundreds of thousands, if not more, lives at stake here, which just absolutely mandates a peaceful settlement through the negotiations that we talked about earlier," Mullen said. "That's what has to happen."