The chairman of the US Joint Chiefs of Staff, Marine Corps General Joseph Dunford, said the intelligence services had not detected any change in Pyongyang’s military stance despite President Donald Trump’s fiery rhetoric against them.
The Republican has repeatedly threatened “fire and fury” against the secretive regime after it stepped up its nuclear and ballistic missile tests in 2017.
Speaking at a White House news conference on Tuesday, Mr Trump said the US was ready with a “military option” which he said will be “devastating”.
“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,” he warned.
Pyongyang has remained defiant and responded to one of Mr Trump’s early threats of military action by saying it was preparing to attack the US-controlled island of Guam in Micronesia.
General Dunford told a Senate Armed Services Committee hearing on his reappointment that “North Korea certainly poses the greatest threat today”.
Although Mr Trump has repeatedly said Pyongyang developing nuclear missiles capable of reaching the US mainland “will not happen”, General Dunford said they will have the capability “soon”.
“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 Korea 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 realise how serious that would be, not only for the United States but for the international community”, he added.
It comes as South Korean politician Lee Cheol-uoo, briefed by the country's spy agency, said North Korea was bolstering its defences by moving aircraft to its east coast and taking other measures after the flight by US bombers.
Mr Lee said the United States appeared to have disclosed the flight route intentionally because North Korea seemed to be unaware.
US Air Force B-1B Lancer bombers, escorted by fighter jets, flew east of North Korea in a show of force last week after the heated exchange of rhetoric between Mr Trump and North Korean dictator Kim Jong-un.
The United States has imposed sanctions on 26 people as part of its non-proliferation designations for North Korea and nine banks, including some with ties to China, the US Treasury Department's Office of Foreign Assets Control Sanctions said on Tuesday.
The US sanctions target people in North Korea and some North Korean nationals in China, Russia, Libya and Dubai, according to a list posted on the agency's website.
Additional reporting by Reuters