The missiles were fired from areas around Hwangju county, in North Hwanghae province, toward the Sea of Japan, South Korea's military said.

Officials said the launch was believed to be of midrange Rodong missiles, which flew about 1,000 kilometers (620 miles) "without giving any prior navigational warning."

North Korean leader Kim Jong Un "expressed great satisfaction" over the launch and called on his country to bolster its nuclear forces, according to a report on Tuesday in the state-run KCNA news agency.

It comes just under two weeks after Pyongyang test-fired a submarine-launched ballistic missile

That missile entered Japan's air defense identification zone, the first time that has happened. Monday's launch also fired missiles into the Japanese defense zone, again without warning.

G20 launch

"Today's reckless launches by North Korea threaten civil aviation and maritime commerce in the region," State Department spokesman John Kirby said on Monday.

"We will raise our concerns at the UN about the threat posed to international security by these programs," he continued. "We will also do so in other fora — including the upcoming East Asia Summit — to bolster international resolve to hold the DPRK accountable for its provocative actions. Our commitment to the defense of our allies in the face of these threats remains ironclad."

Japan's Defense Minister Tomomi Inada said the launch was a "serious threat" to the country's security, adding that it was clear North Korea was learning from past failures.

"Looking at the fact that the three missiles have landed on almost the same spot at almost the same time, I think their missile technology has substantially improved," she said.

Meanwhile, China's foreign minister spokeswoman Hua Chunying called for calm. "At the moment, the situation on the (Korean) peninsula is quite complex and sensitive," Hua said.

"We hope all relevant parties can avoid taking actions that may escalate tensions, and can make joint efforts to maintain peace and stability on the peninsula."

Link to THAAD comments?

According to one analyst, the launches were timed for maximum effect.

"This is Pyongyang's way of reminding everyone of their existence at a moment when all the parties are together, in a typically defiant, North Korean way," John Delury, assistant professor at Yonsei University in South Korea, told CNN.

The launches also came hours after Chinese President Xi Jinping told his South Korean counterpart at the G20 that Beijing opposes the deployment of the United States' THAAD missile defense system to South Korea.

"Mishandling the issue is not conducive to strategic stability in the region and could intensify disputes," Xi was quoted as telling Park Geun-hye, according to a report from China's official Xinhua News Agency.

THAAD woes

The stated purpose of the THAAD system is to defend South Korea from missile attack from the North, but Beijing views it as an attempt to shift the regional power balance in favor of US allies.

Chinese Foreign Minister Wang Yi has previously said the deployment of the anti-missile system could jeopardize "China's legitimate national security interests."

"THAAD has a range that could hit weapons in China," said Yvonne Chiu, an expert on military policy and diplomacy at Hong Kong University.

She added that China was understandably concerned "about having a US made, US run missile system in its backyard."

Delury said that while the Chinese "recognize that something has to be done about North Korea," they view the THAAD system as "killing a chicken with an ax."

While the latest tests may seem to undermine Beijing's anti-THAAD argument, Delury said it "will create more tension between the South Koreans and the Chinese," which will benefit Pyongyang.