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By Phil Helsel

North Korea launched another missile into the sea Wednesday, the latest weapons test amid ratcheted-up tension between the isolated country and the United States.

The South Korean Joint Chiefs of Staff said the missile was launched about 7:50 a.m. (6:50 p.m. ET Tuesday), South Korea's Yonhap News Agency reported. The missile landed in the sea off North Korea's east coast.

Related: South Korea Missile Shield Could Spark New Crisis, Chinese Media Warn

The missile landed in Japan's exclusive economic zone, Japanese defense officials told broadcaster NHK. Defense Minister Gen Nakatani said the missile was believed to be a Rodong, a medium-range missile. He said the missile is believed to have landed 155 miles west of Japan's coast.

"It poses a serious threat to Japan's security and it is an unforgivable act of violence toward Japan's security," Japanese Prime Minister Shinzo Abe told reporters. "This is a clear violation of a U.N. resolution and we lodged a firm protest against North Korea."

Related: North Korea Says It Tested Pre-Emptive Strikes on U.S. Targets

The missile launch was the first since July 19. North Korea has conducted several weapons tests in recent months, and in January it conducted what it claimed was a hydrogen bomb test — although experts have expressed doubts that it was a hydrogen bomb.

North Korea on Feb. 7 launched a long-range rocket that it claimed was for a satellite but which has been called a thinly veiled ballistic missile test. The United States and the United Nations instituted strict new sanctions in response.

This week, North Korea's top diplomat said Washington "crossed the red line" and effectively declared war by putting leader Kim Jong Un on its list of sanctioned individuals.

North Korea has also expressed outrage over the planned deployment of a U.S. missile defense system, the Terminal High Altitude Area Defense, in South Korea.

North Korea had conducted nuclear tests three times before January's test.

A missile is fired during a drill by Hwasong artillery units of the Strategic Force of the Korean People's Army in an image released by North Korea's official news agency on July 21.KCNA via AFP - Getty Images
The Associated Press contributed.