SEOUL, South Korea (AP) — The United States and its Asian allies flew dozens of fighter jets over waters surrounding the Korean Peninsula on Tuesday in a show of force as their diplomats discussed a coordinated response to a possibly imminent North Korean nuclear test.

The flights came as U.S. Deputy Secretary of State Wendy Sherman traveled to Seoul for discussions with South Korea and Japanese officials over the gathering North Korean threat and warned of a “swift and forceful” counterresponse if the North proceeds with a nuclear test explosion, which would be its first in nearly five years.

If staged, the test could be another leap forward in North Korean leader Kim Jong Un’s goal of building an arsenal that can viably threaten regional U.S. allies and the American homeland. That would escalate a pressure campaign aimed at forcing the United States to accept North Korea as a nuclear power and negotiating economic and security concessions from a position of strength.

While the Biden administration has vowed to push for additional international sanctions if North Korea conducts a nuclear test, prospects for robust punitive measures are unclear because of divisions between permanent United Nations Security Council members.

“Any nuclear test would be in complete violation of U.N. Security Council resolutions. There would be a swift and forceful response to such a test,” Sherman said after meeting with South Korean Vice Foreign Minister Cho Hyun-dong. “We continue to urge Pyongyang to cease its destabilizing and provocative activities and choose the path of diplomacy.”

Sherman and Cho are planning a three-way meeting with Japanese Vice Foreign Minister Mori Takeo on Wednesday over the North Korean nuclear issue.

Extending the countries’ joint displays of military might, four U.S. F-16 fighter jets flew in formation with 16 South Korean planes — including F-35A stealth fighters — over waters off South Korea’s eastern coast, an exercise aimed at demonstrating an ability to quickly respond to North Korean provocations, South Korea’s Joint Chiefs of Staff said. The United States and Japan conducted a separate drill involving six aircraft — four Japanese F-15 fighters and two American F-16s — above waters between the Korean Peninsula and Japan, Japan’s Defense Ministry said.

The flights came a day after U.S. and South Korean forces fired eight surface-to-surface missiles into South Korea’s eastern waters to match a weekend missile display by North Korea, which fired the same number of weapons from multiple locations Sunday in what was likely its biggest single-day testing event.

North Korea has conducted 18 rounds of missile launches in 2022 alone — including its first demonstrations of intercontinental ballistic missiles since 2017 — exploiting a favorable environment to push forward weapons development, with the Security Council effectively paralyzed over Russia’s war on Ukraine.

North Korea may soon up the ante as U.S. and South Korean officials say it is all but ready to conduct another detonation at its nuclear testing ground in the northeastern town of Punggye-ri. Its last such test and sixth overall was in September 2017, when it claimed to have detonated a thermonuclear bomb designed for its ICBMs.

Since taking power in 2011, Kim has accelerated nuclear weapons development despite limited resources and has shown no willingness to fully surrender an arsenal he sees as his strongest guarantee of survival.

Experts say with its next nuclear test, North Korea could claim an ability to build small bombs that could be placed on a multiwarhead ICBM or fit on short-range missiles that could reach South Korea and Japan.

Rafael Mariano Grossi, director general of the International Atomic Energy Agency, said Monday there are indications that one of the passages at the Punggye-ri testing ground has been reopened, possibly in preparation for a nuclear test.

Hours before Sherman’s meeting in Seoul, State Department spokesperson Ned Price told reporters in Washington that the United States believes North Korea could seek its seventh test “in the coming days.”

The Biden administration’s punitive actions over North Korea’s recent weapons tests have been limited to largely symbolic unilateral sanctions. Russia and China vetoed a U.S.-sponsored resolution in the Security Council that would have imposed additional sanctions on North Korea over its previous ballistic tests on May 25.

“We have called on members of the international community, certainly members of the U.N. Security Council’s permanent five, to be responsible stakeholders in the U.N. Security Council as a preeminent forum for addressing threats to international peace and security,” Price said.

“Unilateral actions are never going to be the most attractive or even the most effective response, and that is especially the case because we are gratified that we have close allies in the form of Japan and the ROK,” he said, referring to South Korea’s formal name, the Republic of Korea.

North Korea’s state media have yet to comment on Sunday’s launches. They came after the U.S. aircraft carrier Ronald Reagan concluded a three-day naval drill with South Korea in the Philippine Sea on Saturday, apparently their first joint drill involving a carrier since November 2017, as the countries move to upgrade their defense exercises in the face of growing North Korean threats.

North Korea has long condemned the allies’ combined military exercises as invasion rehearsals and has often countered with its own missile drills, including launches in 2016 and 2017 that simulated nuclear attacks on South Korean ports and U.S. military facilities in Japan.

Nuclear talks between the U.S. and North Korea have stalled since 2019 because of disagreements over an easing of crippling U.S.-led sanctions in exchange for North Korean disarmament steps.

Kim’s government has so far rejected the Biden administration’s offers of open-ended talks, and is clearly intent on converting the dormant denuclearization negotiations into a mutual arms-reduction process, experts say.

Kim’s pressure campaign hasn’t been slowed by a COVID-19 outbreak spreading across his largely unvaccinated populace of 26 million amid a lack of public health tools. North Korea has so far rejected U.S. and South Korean offers of help, but there are indications that it received at least some vaccines from ally China.

South Korean activist Park Sang-hak, a North Korean defector who for years has launched anti-Pyongyang propaganda leaflets by balloon across the border, said his group on Tuesday flew 20 balloons carrying medicine, masks and vitamin pills to help North Korean civilians.

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Associated Press writer Mari Yamaguchi in Tokyo contributed to this report.

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