"We are totally prepared for the second option, not a preferred option," Trump said at a White House news conference on Tuesday, referring to military force.
"But if we take that option, it will be devastating, I can tell you that, devastating for North Korea. That's called the military option. If we have to take it, we will."
Bellicose statements by Trump and North Korean leader Kim Jong Un in recent weeks have created fears that a miscalculation could lead to action with untold ramifications, particularly since Pyongyang conducted its sixth and most powerful nuclear test on September 3.
Despite the escalation in tensions, the United States has not detected any change in North Korea's military posture reflecting an increased threat, the top US military officer said on Tuesday.
The assessment by Marine Corps General Joseph Dunford, chairman of the US Joint Chiefs of Staff, about Pyongyang's military stance was in contrast to a South Korean lawmaker who said Pyongyang had boosted defenses on its east coast.
"While the political space is clearly very charged right now, we haven't seen a change in the posture of North Korean forces, and we watch that very closely," Dunford told a Senate Armed Services Committee hearing on his reappointment to his post.
In terms of a sense of urgency, "North Korea certainly poses the greatest threat today," Dunford testified.
North Korean Foreign Minister Ri Yong Ho on Monday accused Trump of declaring war on the North and threatened that Pyongyang would shoot down US warplanes flying near the peninsula after American bombers flew close to the Korean peninsula last weekend.
Ri was reacting to Trump's Twitter comments that Kim and Ri "won't be around much longer" if they acted on their threats toward the US.
North Korea has been working to develop nuclear-tipped missiles capable of hitting the US mainland, which Trump has said he will never allow. Dunford said Pyongyang will have a nuclear-capable intercontinental ballistic missile "soon," and it was only a matter of a "very short time".
"We clearly have postured our forces to respond in the event of a provocation or a conflict," the general said, adding that the United States has taken "all proper measures to protect our allies" including South Korean and Japan.
"It would be an incredibly provocative thing for them to conduct a nuclear test in the Pacific as they have suggested, and I think the North Korean people would have to realize how serious that would be, not only for the United States but for the international community," Dunford said.
South Korean lawmaker Lee Cheol-uoo, briefed by the country's spy agency, said North Korea was bolstering its defenses by moving aircraft to its east coast and taking other measures after the flight of the US bombers. Lee said the United States appeared to have disclosed the flight route of the bombers intentionally because North Korea seemed to be unaware.
The US has imposed sanctions on 26 individuals as part of its non-proliferation designations for North Korea as well as nine banks, including some with ties to China, according to the US Treasury Department's Office Of Foreign Assets Control Sanctions.
The US sanctions target individuals in North Korea as well as some North Korean nationals in China, Russia, Libya and Dubai.
'CAPABILITY TO DETER'
During a visit to India, US Defense Secretary Jim Mattis said diplomatic efforts continued.
"You have seen unanimous United Nations Security Council resolutions passed that have increased the pressure, economic pressure and diplomatic pressure, on the North, and at the same time, we maintain the capability to deter North Korea's most dangerous threats," he told reporters in the Indian capital.
Speaking in Beijing, Chinese Foreign Ministry spokesman Lu Kang said war on the Korean peninsula would have no winner.
"We hope the US and North Korean politicians have sufficient political judgment to realize that resorting to military force will never be a viable way to resolve the peninsula issue and their own concerns," Lu said.
"War on the peninsula will have no winner."
South Korean President Moon Jae-in urged Kim Jong Un to resume military talks and reunions of families split by the 1950-53 Korean War to ease tension.
"Like I've said multiple times before, if North Korea stops its reckless choices, the table for talks and negotiations always remains open," Moon said.
In Moscow, Russia's Foreign Ministry said it was working behind the scenes to find a political solution and that it plans to hold talks with a representative of North Korea's foreign ministry who is due to arrive in Moscow on Tuesday, the RIA news agency cited the North's embassy to Russia as saying.
US Air Force B-1B Lancer bombers, escorted by fighter jets, flew east of North Korea in a show of force after the heated exchange of rhetoric between Trump and Kim.
The United States and South Korea are technically still at war with North Korea after the 1950-53 conflict ended in a truce and not a peace treaty.
