North Korea fired two short-range ballistic missiles into the ocean hours after US Vice President Kamala Harris flew home from a visit to South Korea.
She emphasised Washington's "ironclad" commitment to the security of its Asian allies as she visited the Demilitarised Zone (DMZ) separating the two Koreas.
It was the third round of missile launches by North Korea this week, extending a record pace of weapons testing in the rogue state.
South Korea’s joint chiefs of staff said the missiles were fired nine minutes apart from an area just north of the North's capital, Pyongyang - landing in the sea of Japan.
Japanese Defence Minister Yasukazu Hamada said the launches "one-sidedly escalate provocation" and are "absolutely impermissible".
North Korea also fired two short-range ballistic missiles on Wednesday, while Ms Harris was in Japan, and 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 DMZ, where she addressed the threat posed by the increasingly hostile North.
There are concerns that North Korea may soon conduct a nuclear test, which would move the country closer to being acknowledged as a full-fledged nuclear power.
Last week South Korean officials said they had detected signs North Korea was preparing to test a ballistic missile system designed to be fired from submarines. Visiting the heavily guarded DMZ has become something of a ritual for American leaders hoping to show their resolve to stand firm against aggression.
Surveying the site with a bulky pair of binoculars, Ms Harris said "it's so close" as she looked at the barbed-wire fences and claymore mines along the military demarcation line, which American soldiers patrol on the southern side.
The vice president visited one of a row of blue buildings that straddle the demarcation line, which are still used to conduct negotiations with North Korea.
Sometimes they pass messages back and forth and sometimes they use a megaphone, a US officer said.
On the northern side of the demarcation line, two figures dressed in what appeared to be hazmat suits peeked out from behind a curtain from a second-floor window then disappeared back inside.
Ms Harris described this week's missile launches as provocations meant to “destabilise the region" and said the United States and South Korea remain committed to the “complete denuclearisation" of the North.
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“I cannot state enough that commitment of the United States to the defence of the Republic of Korea is ironclad,” she said. “In the South, we see a thriving democracy. In the North, we see a brutal dictatorship,” she said before flying away from the border on a US military helicopter. Earlier, Ms Harris met with Mr Yoon at his office in Seoul and reaffirmed the US' commitment to defend the South with a full range of its military capabilities in the event of war.
Mr Yoon's office expressed concern over Pyongyang's threats of nuclear conflict and pledged an unspecified stronger response to major North Korean provocations, including a nuclear test.
Ms Harris and Mr Yoon were also expected to discuss repairing recently strained ties between South Korea and Japan to strengthen their trilateral cooperation with Washington in the region.
Their meeting also touched on Taiwan, amid fears of China's increasingly hostile rhetoric on the island democracy, which Beijing claims as its own territory.
Both the US and South Korea reaffirmed their support for “peace and stability” in the Taiwan Strait, Mr Yoon’s office said, without elaborating further. Ms Harris’ trip was organised so she could attend the state funeral of former Japanese Prime Minister Shinzo Abe, but her itinerary was dominated by security concerns in the region.
In every meeting, Ms Harris tried to lay to rest any fears that the United States was wavering in its commitment to protect its allies, describing American partnerships with South Korea and Japan as the “linchpin” and “cornerstone” of its defence strategy in Asia.