North Korea fired two missiles from a submarine in the East Sea, as the Sea of Japan is also known, over the weekend, North Korean state media said on Monday.
State-controlled KCNA said two "strategic cruise missiles" were fired from the submarine 8.24 Yongung in an "underwater launching drill" on Sunday morning.
The submarine was in the waters off Kyongpho Bay and "precisely hit the preset target" in the East Sea after traveling the 1,500 kilometres in "eight-shaped flight orbits" for over two hours, KCNA reported.
The agency said that Pyongyang was monitoring the situation in the Korean peninsula and "in which the U.S. imperialists and the south Korean puppet forces are getting evermore undisguised in their anti-DPRK military maneuvers."
On Monday, South Korea and the US were set to kick off another major exercise dubbed "Freedom Shield."
South Korea's Joint Chiefs of Staff (JCS) earlier had said that it detected an unspecified missile launch from a submarine in waters off North Korea's eastern coastal city of Sinpo, without immediately providing other details, news agency Yonhap reported.
On Friday, North Korea's state-controlled media cited ruler Kim Jong Un as saying he wants to prepare his country's army for a possible war on the Korean peninsula and is intensifying military training to improve readiness.
Kim supervised firing exercises of an artillery unit on the "western front" to check its current readiness for war, the report said. This included simulating strikes against enemy airports on Thursday by firing a "powerful salvo" towards the sea, it said.
No countries were named in the report. However, North Korea, which is largely isolated internationally because of its nuclear weapons programme, sees neighbouring South Korea and the US as enemies.
Tensions on the Korean peninsula have increased considerably again since last year. North Korea is increasingly testing nuclear-capable missiles, despite a ban by UN resolutions. The US and South Korea have meanwhile resumed full-scale joint military exercises.
