SEOUL — Buttressed by the success of South Korea’s ruling conservative party in local elections, Seoul and Washington are warning North Korea about its missile and nuclear weapons program and threats against the South. 

The veteran American envoy on North Korea, Sung Kim, strongly suggested Washington would increase its military role in the region in direct response to North Korean “provocations.”

At the opening of talks with his opposite numbers from South Korea and Japan, Mr. Kim said bluntly, “We are preparing … to make both short- and long-term adjustments to our military posture” in response to any “provocation and as necessary to strengthen both defense and deterrence and to protect our allies.”

The message was unmistakable: Although the United States only has 28,500 troops in South Korea, Washington is ready to send in reinforcements if needed and also to send warplanes on intimidation missions as messages to North Korea’s leader, Kim Jong-un.

South Korea’s nuclear envoy, Kim Gunn, opening the session, was just as emphatic. “North Korea’s relentless pursuit of nuclear weapons can only strengthen our deterrence,” he said. Korea along with Japan and the United States, he added, “would come up with a united response.”

In a not-so-subtle reminder of the possibility of a military response, he warned that “prolonged isolation can only worsen the dire situation.”

The remarks of the two envoys marked a fresh determination to stand firm against the threats of North Korea, a sharp departure from the attempts at reconciliation and appeasement by South Korea’s previous president, Moon Jae-in.

They reflected a measure of confidence that was buoyed by local elections on Wednesday in which candidates of the ruling People Power Party won12 of the 17 positions as governors of the country’s eight provinces or mayors of nine cities, including the capital and Korea’s second-biggest city, Busan.

There was no doubt also that the recent summits between President Biden and the leaders of both South Korea and Japan had strengthened “trilateral cooperation” despite the deep reluctance of both countries to form an alliance.

Now the three countries are engaged in discussions “on all levels,” Korea’s Kim Gunn said.

Facing him from his side of three tables formed to make a triangle in South Korea’s foreign ministry, Japan’s Takehiro Funakoshi said “trilateral cooperation is all the more important” considering that “further provocations including a nuclear test are possible.” Mr. Biden’s visits, he said, “highlighted our mutual deterrence in the region.”

The trilateral talks on Friday were just the beginning of a campaign to impress on North Korea the determination of the United States, Korea, and Japan to forestall whatever incendiary plans the North Korean strongman may have in mind to distract attention from the Covid-19 pandemic that’s raging throughout his country.

Washington, which has long held separate alliances with South Korea and Japan, is attempting to get Seoul and Tokyo to join in military exercises and other displays of goodwill even though the legacy of the Japanese colonial era in Korea presents obstacles — enough to keep the two from forming a formal alliance.

The pressure for trilateral cooperation reaches a new level next week when a deputy secretary of state, Wendy Sherman, arrives here for talks with her vice-ministerial counterparts from Seoul and Tokyo. In effect, Sung Kim, a highly experienced negotiator who’s previously served as ambassador to South Korea and the Philippines and is now concurrently ambassador to Indonesia, was laying the groundwork for Ms. Sherman’s visit.

Washington appears anxious to capitalize expeditiously on the wave of support for the new conservative president. The sense of relief after problems engendered by the former president’s soft-line toward Pyongyang is almost palpable.

“Our bottom line has not changed,” Sung Kim said. That is “to make clear” that North Korea’s “unlawful activities have consequences.”