As North Korea test-launches missiles, leader's sister says country sees 'urgency' in building military might

SEOUL, TOKYO and LONDON -- North Korea test-launched several short-range ballistic missiles on Tuesday morning, neighboring South Korea and Japan said.

South Korea's military "has detected and is analyzing several short-range ballistic missiles launched into the East Sea" from North Korea at around 7:30 a.m. local time, South Korea's Joint Chiefs of Staff confirmed to ABC News.

"Our military has strengthened surveillance and vigilance in preparation for additional launches and is maintaining a full readiness posture while closely sharing information related to North Korea's ballistic missiles with the U.S. and Japanese authorities," the military said.

The launches come days after a trilateral military exercise run by the United States, South Korea and Japan, according to the U.S. Indo-Pacific Command.

Kim Yo Jong, North Korean leader Kim Jong Un's sister, called those exercises "another clear behavioral explanation of the most hostile and dangerous aggressive nature," according to the Korean Central News Agency, a North Korean state media service.

Kim Yo Jong said those exercises also justified North Korea's nuclear strategy, stating they prove "the legitimacy and urgency of the line of strengthening nuclear forces that we have chosen and are implementing," according to KCNA.

U.S. military officials said on Tuesday they were aware of the North Korean launches, along with the test-launch of an intercontinental ballistic missile last week. The Indo-Pacific Command in a statement called the launches "destabilizing acts."

"While we have assessed that this event does not pose an immediate threat to U.S. personnel, or territory, or to our allies, we continue to monitor the situation," the statement said. "The U.S. commitments to the defense of [South Korea] and Japan remain ironclad."

Japan’s Ministry of Defense said the multiple short-range ballistic missiles were all believed to have fallen outside Japan's exclusive economic zone. The missiles were launched toward the northeast from North Korea’s west coast between 7:30 a.m. and 7:39 a.m., the ministry said.

The missiles flew about 400 km, or 250 miles, South Korea's Joint Chiefs of Staff said.

South Korea's military detected the missiles upon launch, then tracked, and monitored them, while sharing relevant information with the U.S. and Japan. Detailed specifications were being analyzed, South Korea's Joint Chiefs of Staff said.

Several of the missiles fired on Tuesday were estimated to have reached a maximum altitude of about 62 miles, Japan said. No damage to ships or aircraft was reported.

Japan’s Ministry of Defense in Tokyo vowed to remain vigilant, to continue to gather information and to monitor the situation closely.

As Prime Minister Shigeru Ishiba entered his just before 9 a.m., he told reporters, “No information on damage has been confirmed at this time. In response, I have issued instructions to provide information to the public and remain vigilant.”

ABC News' Ellie Kaufman contributed to this report.