Melanie Eversley, USA TODAY 8:22 a.m. EDT August 24, 2016

North Korea fired a submarine-based missile toward Japan Wednesday, CNN, Reuters and other news organizations are reporting, attributing the word to South Korea's Joint Chiefs of Staff and its Defense Ministry.

The missile was fired at around 5:30 a.m. local time from the city of Sinpo on North Korea's east coast, Reuters reported.

The act is one of a series of launches conducted by the country in defiance of United Nations sanctions.

The launch comes during an annual drill between the United States and South Korea that involves 25,000 U.S. troops, CNN reported. North Korea has called the exercises preparation for invasion and has threatened retaliation, according to Reuters.

The missile traveled more than 300 miles toward Japan and U.S. Pacific Command tracked it into the Sea of Japan, CNN reported.

The launch was believed to be "part of North Korea's armed protest to escalate military tension in the Korean Peninsula using annual U.S.- South Korea joint drill as an excuse," read a statement from the South Korea Joint Chiefs of Staff, CNN reported.

The launch appeared to be North Korea's most successful attempt so far at launching a submarine-based ballistic missile, the BBC reported. Such launches are considered particularly tricky because preparation for launch can be difficult to detect, according to the BBC.

Though the missile went down into the sea, it penetrated an air defense security zone outside of Japan, the BBC reported.