North Korea fired two "strategic cruise missiles" from a submarine in waters off its east coast over the weekend, state media said Monday, in apparent protest over a regular South Korea-US combined military exercise to begin this week.

The missiles were fired from the 8.24 Yongung in waters off Kyongpho Bay in the East Sea in an underwater launching drill held at dawn Sunday, according to the official Korean Central News Agency.

"The drill confirmed the reliability of the weapon system and examined the underwater-to-surface offensive operations of submarine units that constitute one of other major forces of the DPRK nuclear deterrent," the KCNA said in an English-language report, using the acronym of the North's official name.

The missiles precisely hit preset targets in the East Sea after "traveling the 1,500 km-long eight-shaped flight orbits for 7,563 to 7,575 seconds."

"The drill...verified the current operation posture of the nuclear war deterrence means in different spaces," the KCNA said.

Earlier in the day, South Korea's military said it detected an unspecified missile launched from a submarine in waters off the North's eastern coastal city of Sinpo on Sunday morning. It did not immediately provide other details.

"While strengthening its monitoring and vigilance, our military is maintaining a full readiness posture in close cooperation with the United States," the Joint Chiefs of Staff said in a text message sent to reporters.

The latest launch came on the eve of the allies' major Freedom Shield exercise, which kicks off Monday. The North has decried the exercise as "preparations for a war of aggression" against it.

At a key military meeting presided over by North Korean leader Kim Jong-un, the North has decided to take "important, practical" measures for the "offensive use" of war deterrents, according to the KCNA. (Yonhap)