North Korean leader Kim Jong Un is seen during the test fire of a strategic submarine-launched ballistic missile in an undated photo released by North Korea's Korean Central News Agency in August. (KCNA/via Reuters)

North Korea fired three medium-range ballistic missiles into the sea off its east coast Monday, landing close to Japan, in a show of force that coincided with the meeting of leaders of the world’s 20 largest economies in neighboring China.

North Korea launched the missiles, believed to be Rodongs, from a site south of Pyongyang at 12:14 p.m. local time, South Korea’s military said. They flew about 600 miles and landed well inside Japan’s air defense identification zone, the area in which Tokyo controls aircraft movement.

The launches, coming as the G-20 meeting continued in the eastern Chinese city of Hangzhou and just days before North Korea marks the 68th anniversary of the formation of its government, constituted an “armed protest,” an official from South Korea’s joint chiefs of staff said in a statement.

“We are fully prepared to fight tonight in case North Korea makes any provocative moves,” the statement said, according to the Yonhap News Agency, using the catchphrase of the American and South Korean military allies.

Japan’s Defense Ministry added that the missiles landed between 120 and 160 miles west of Hokkaido, the northernmost of Japan's four main islands.

South Korea announced on Sept. 5 that North Korea fired three ballistic missiles in an act of defiance as world leaders met in China for the G20 Summit. (Reuters)

At the G-20 meeting, South Korean President Park Geun-hye and Japanese Prime Minister Shinzo Abe met immediately and agreed to cooperate against North Korea.

In Washington, State Department spokesman John Kirby said the United States “strongly condemns” the launches.

“These launches, which have become far too common in the past several months, violate multiple UN Security Council Resolutions explicitly prohibiting North Korea’s launches using ballistic missile technology,” he said in a statement, adding that U.S. officials would raise the issue during the meeting of the Association of Southeast Asian Nations in Laos on Tuesday.

“Our commitment to the defense of our allies in the face of these threats remains ironclad,” Kirby said.

Monday’s launches were just the latest salvo in a steady series of missiles coming from North Korea. Last month, Kim Jong Un’s regime claimed a “great success” in launching a ballistic missile from a submarine about 300 miles toward Japan, on top of making progress on its medium-range Musudan missile technology.

This is a particularly tense time in the region because of frictions over the Terminal High Altitude Area Defence (THAAD) anti-missile battery that the United States will deploy to South Korea, part of their defense against North Korea.

Beijing has protested strongly against the plan, viewing it as part of an American effort to restrain a strengthening China, and worrying that the system will home in on China’s military activities.

The issue has helped close the gap between China and its erstwhile client state, North Korea, after the provocations of a nuclear test and long-range missile launch earlier this year. Beijing and Pyongyang have traditionally been “as close as lips and teeth,” as the saying goes, but Xi Jinping, China’s president, has made his disdain for the young Kim clear.

Earlier in the day, during a bilateral meeting at the G-20 with Xi, Park said she hoped Seoul and Beijing would be able to unite together against North Korea.

I “hope that through earnest communication, our two countries can turn this challenge into an opportunity to further strengthen and move forward our bilateral relationship,” Park said during a bilateral meeting with Xi, according to Yonhap.

But Xi reiterated his strong objections to Park’s decision to accept the THAAD battery onto South Korean soil.

“Mishandling the issue is not conducive to strategic stability in the region and could intensify disputes,” Xi told Park, according to a report from the Chinese state-run news agency Xinhua.