PANMUNJOM, Korea -- In a show of defiance, North Korea fired at least one ballistic missile into the sea on Thursday, hours after U.S. Vice President Kamala Harris flew home from a visit to South Korea during which she emphasized the “ironclad” U.S. commitment to the security of its Asian allies.

It was the third round of missile launches by North Korea this week, extending a record pace in weapons testing as it accelerates a push to expand its arsenal and pressure Washington to accept it as a nuclear power.

South Korea’s Joint Chiefs of Staff didn’t immediately confirm what type of missile was launched Thursday or how far it flew. Japan's military said it also detected a launch. North Korea fired two short-range ballistic missiles on Wednesday, while Harris was in Japan, and fired one before she left Washington on Sunday.

Harris earlier capped her four-day trip to Asia with a meeting with South Korean President Yoon Suk Yeol and a stop at the Demilitarized Zone dividing the Korean Peninsula, where she addressed the threat posed by the increasingly hostile North.

There are also concerns that North Korea may conduct a nuclear test, which would move the country closer to being acknowledged as a full-fledged nuclear power. Visiting the DMZ has become something of a ritual for American leaders hoping to show their resolve to stand firm against aggression.

At the DMZ, Harris went to the top of a ridge, near guard towers and security cameras. She looked through bulky binoculars as a South Korean officer pointed out military installations on the southern side. Then an American officer pointed out some of the defenses along the military demarcation line, including barbed-wire fences and claymore mines. He said American soldiers regularly walk patrols along a path.

Harris then visited one of a row of blue buildings that straddle the demarcation line, where an American officer explained how the buildings are still used to conduct negotiations with North Korea. Sometimes they pass messages back and forth and sometimes they use a megaphone, he said.

“That’s high tech,” Harris joked, before adding, “We’ve stepped into history.”

“It’s still going,” the colonel said.

Harris agreed. “The past and present are happening every day.”