U.S. Secretary of State Rex Tillerson said diplomatic efforts to rein in Pyongyang’s nuclear weapons program “will continue until the first bomb drops,” as the U.S. and South Korean navies kicked off a massive, five-day joint military exercise Monday in the waters surrounding North Korea.

The statement by America’s top diplomat came despite a series of recent tweets by President Donald Trump that seemed to rule out diplomacy, including one that said Tillerson was “wasting his time trying to negotiate with Little Rocket Man,” a mocking reference to North Korean leader Kim Jong Un.

Speaking Sunday on CNN’s “State of the Union,” Tillerson was asked if Trump’s tweets had undermined any diplomatic push for solving the North crisis.

“Well, I think the president — what the president is doing is, he’s trying to motivate action on a number of people’s part, in particular the regime in North Korea,” Tillerson said.

“I think he does want to be clear with Kim Jong Un and that regime in North Korea that he has military preparations ready to go and he has those military options on the table. And we have spent substantial time actually perfecting those.”

Trump has taken a hard-line approach with North Korea, variously threatening it with “fire and fury” and to “totally destroy” the country of 25 million people if the United States is forced to defend itself or its allies, including Japan.

His bellicose remarks — including those about “the calm before the storm” and only “one option” for denuclearizing the North — have prompted some to believe that Trump may be leaning toward a military strike against the Kim regime. Experts have warned that such a move would have potentially devastating results for Northeast Asia and beyond.

Tillerson, however, reiterated earlier statements saying that Trump was committed to diplomacy.

“The president,” he said, “has also made clear to me that he wants this solved diplomatically. He’s not seeking to go to war.

Trump, Tillerson added, “has made it clear to me to continue my diplomatic efforts, which we are, and we will — as I have told others, those diplomatic efforts will continue until the first bomb drops.”

Still, Trump’s heated rhetoric, the U.S. and allied military muscle-flexing near North Korea, and Pyongyang’s barrage of missile and nuclear tests have combined to push up tensions to levels unseen in recent years.

The large-scale military drills, which were due to run through Friday, involve fighter jets, helicopters and 40 naval ships, including the Yokosuka, Kanagawa Prefecture-base aircraft carrier USS Ronald Reagan, the South Korean Navy said.

The South Korean military said the drills are aimed at practicing how to respond to a potential naval provocation by North Korea and improving the allies’ combined operational capability, according to media reports. The drills were to include live-fire exercises by warships and aircraft, and anti-submarine training.

Washington has ramped up its deployment of military assets to the region, announcing on Friday that the nuclear-powered submarine USS Michigan, which is armed with Tomahawk cruise missiles, had arrived from Guam at the South Korean port of Busan “for a routine visit.” And on Tuesday, the U.S. sent B-1B heavy bombers from Guam for exercises with the South Korean Air Force over the Korean Peninsula

North Korea despises the exercises and shows of force, calling them a rehearsal for invasion.

In August, North Korea stoked concern when it unveiled a plan, which it later backed away from, to fire a salvo of intermediate-range missiles to bracket the island of Guam — a key U.S. military outpost — with “enveloping fire.”

On Friday, state-run media hinted in a commentary that moves by the U.S. military in the region could prompt a rethink of that plan.

“We have already warned several times that we will take counteractions for self-defense, including a salvo of missiles into waters near the U.S. territory of Guam, an advance base for invading the DPRK,” Kim Kwang Hak, a researcher with the North Korean Foreign Ministry’s Institute for American Studies warned in the commentary. DPRK is the acronym for the North’s formal name, the Democratic People’s Republic of Korea.

“The U.S. military action hardens our determination that the U.S. should be tamed with fire and lets us take our hand closer to ‘trigger’ … the toughest countermeasure.”

A missile-firing near Guam would be extremely provocative and risks a miscalculation from either side that could erupt into military conflict.

Elsewhere, U.S. Ambassador to the United Nations Nikki Haley responded to growing concerns that the possibility of the U.S. pulling out of the hard-fought nuclear deal with Iran could have damaging repercussions for any attempt at dialogue with Pyongyang.

Asked it such a move would affect Washington’s credibility at the negotiating table, Haley was adamant that would not be the case.

“It sends the perfect message to North Korea, which is, we’re not going to engage in a bad deal,” Haley told ABC News’ “This Week” on Sunday. “Should we ever get into a deal, we’re going to hold you accountable. We’re not going to look the other way, just because we think we have made a deal and we’re not going to continue to watch it. We have to watch it.”

North Korea has made a series of breakthroughs in its nuclear and missile programs this year, conducting its sixth and most powerful nuclear test and sending intermediate-range missiles twice over Japan in recent months.

Kim has overseen a spate of tests to master the technology needed to reliably target the United States with a nuclear-tipped long-range missile, conducting dozens of test-firings and training launches since last year. In July, it twice tested an intercontinental ballistic missile that experts say is capable of striking a large chunk of the U.S.