BANGKOK (AP) — U.S. Defense Secretary Jim Mattis is visiting the Korean Peninsula at a momentous juncture in the faltering effort to persuade Pyongyang to halt and dismantle its nuclear weapons program. Ominous questions hang in the air.

Is diplomacy failing? Is war approaching?

Mattis' second trip as Pentagon boss to Seoul will take place Friday, following his consultations with Asian partners on a unified approach to resolve the North Korea crisis.

In the Philippines, his Japanese counterpart spoke darkly of an "unprecedented, critical and imminent" threat posed by the North's repeated demonstrations of its ability to launch an intercontinental-range missile, potentially armed with a nuclear warhead. Twice, in August and September, North Korean missiles overflew Japan's northern Hokkaido island, triggering alarms and warnings for citizens to take cover.

As North Korea's capabilities rush toward putting the U.S. mainland in range, Mattis has stuck to the American diplomacy and pressure campaign led by Secretary of State Rex Tillerson. The goal is to compel the North to a complete and irreversible removal of its nuclear arsenal.

"Everyone is out for a peaceful resolution. No one's rushing for war," Mattis told reporters Wednesday on a flight to Thailand. From there, he is traveling on to South Korea.

But there are increasing suggestions of possible military confrontation. Trump's national security adviser, Lt. Gen. H.R. McMaster, said last week, "We are in a race to resolve this short of military action," adding, "We are running out of time."

Michael Swaine, a longtime Asia specialist at the Carnegie Endowment for International Peace, said that while he is hopeful of averting conflict, "I don't see any clear signs that there is progress in either coercing the North Koreans into starting to talk about denuclearization or finding some other path toward some kind of engagement with North Korea."

"Recent months have shown a worsening of the relationship between the U.S. and North Korea that is very troubling to me," he said in an interview. "I'm concerned about the president's upcoming trip to Asia where the North Koreans could use this as an opportunity to conduct some additional test."

President Donald Trump will visit South Korea next month. Aides say he will not travel to the Demilitarized Zone, the internationally recognized buffer zone that has separated the two Koreas since the Korean War. The fighting ended in 1953 with an armistice, not a peace treaty, meaning the United States and North Korea are still technically at war.

Trump has mocked North Korean leader Kim Jong Un as "Little Rocket Man" and threatened to unleash "fire and fury" on Pyongyang if its leaders do not abandon their nuclear weapons.

Kim seems undaunted by threats and unresponsive to diplomatic overtures. He has traded insults with Trump and kept his country marching -- some say speeding -- toward a capability to strike any American city with a nuclear weapon. Trump has said he will never allow the North to reach that point.

In Seoul, Mattis will attend annual meetings Saturday with senior South Korean government officials and assess plans for countering the North's threats. He'll also reaffirm America's promise to defend the South against any attack, and possibly discuss the outlook for giving the South wartime operational control of its own forces.

The U.S. has about 28,500 troops in South Korea, including at Osan air base where the Air Force maintains fighter aircraft. More than a decade ago, the U.S. was prepared to give Seoul operational control of South Korean forces in the event of war with the North, but the U.S. ally has repeatedly asked that the transition be delayed. In 2014, the sides agreed to drop any timetable and hand off control only when both decide conditions are right. Thus, U.S. Army Gen. Vincent K. Brooks, who commands all U.S. troops in Korea, also would be in charge of South Korean troops if war broke out tomorrow.

The North's Kim has vowed to complete his country's development of a nuclear arsenal, a project begun by his grandfather, Kim Il Sung, in defiance of international condemnations and United Nations economic sanctions. Even China, the North's traditional benefactor, has taken stronger economic measures to pressure the North to return to negotiations.

None of the pressure has worked as the North insists a nuclear arsenal with global reach protects it from what it sees as U.S. efforts to overthrow the government.

Choe Son-hui, a senior Foreign Ministry official, told a conference in Moscow last week that his country will develop nuclear weapons and missiles until achieving a "balance of power" with the United States. Conference participants recounted her saying the nukes were non-negotiable unless Washington ended its "hostile policy."

The U.S. has stepped up the tempo of military exercises with allies, including periodic flights by strategic bombers over the peninsula and naval drills with South Korea last week. The activity has raised questions about whether Washington is showing force to deter Pyongyang or readying for a conflict.

After North Korea conducted a series of ballistic missile tests and an underground nuclear test in September that the North said was a hydrogen bomb, it has kept the world guessing on what it will do next.

If it again launches a missile through Japanese airspace, will Japan or the U.S. attempt to shoot it down? Will the North detonate a nuclear bomb over the Pacific, as Kim's foreign minister recently suggested? And could that presage war?

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AP writer Matthew Pennington in Washington contributed to this report.