North Korea test-fired two strategic cruise missiles from a submarine in a show of force hours before the US and South Korea were to stage major joint military exercises, state media reported early Monday.
A submarine fired the weapons from waters off the eastern coastal city of Sinpo on Sunday morning, the KCNA news agency said.
The South Korean military said it detected the launch of a single unspecified missile, without giving details, the Yonhap news agency said.
KCNA said the drill was successful, as the missiles hit their designated and unspecified targets in waters off the east coast of the Korean Peninsula.
The launch came hours before South Korea and the US were set to kick off their largest joint exercises in five years on Monday. Nuclear-armed Pyongyang has warned such drills could be seen as a "declaration of war".
The KCNA report announcing Sunday's missile launch said the test firing expressed "the invariable stand" of North Korea to confront a situation in which "the US imperialists and the South Korean puppet forces are getting ever more undisguised in their anti-DPRK military maneuvers."
DPRK is the acronym for North Korea's official name.
KCNA said the drill also "verified the current operation posture of the nuclear war deterrence means in different spaces."
In a separate statement, North Korea's foreign ministry said the US was "scheming" to call a UN Security Council meeting on human rights in the reclusive communist state, to coincide with the joint maneuvers.
"The DPRK bitterly denounces the US vicious 'human rights' racket as the most intensive expression of its hostile policy toward the DPRK and categorically rejects it," the ministry said, according to KCNA.
Washington and Seoul have ramped up defence cooperation in the face of growing military and nuclear threats from the North, which has conducted ever more provocative banned weapons tests in recent months.
The US-South Korea exercises, called Freedom Shield, are scheduled to run for at least 10 days from Monday and will focus on the "changing security environment" due to North Korea's redoubled aggression, the allies said.
In a rare move, the Seoul military this month revealed that it and Washington special forces were staging "Teak Knife" military exercises – which involve simulating precision strikes on key facilities in North Korea – ahead of Freedom Shield.
All such exercises infuriate North Korea, which views them as rehearsals for an invasion.
It has said its nuclear weapons and missile programmes are for self-defence.
"Pyongyang has military capabilities under development it wants to test anyway and likes to use Washington and Seoul's cooperation as an excuse," said Leif-Eric Easley, a professor at Ewha University in Seoul.
Last year, the North declared itself an "irreversible" nuclear power and fired a record-breaking number of missiles, with leader Kim Jong Un last week ordering his military to intensify their own drills to prepare for a "real war".
Washington has repeatedly restated its "ironclad" commitment to defending South Korea, including using the "full range of its military capabilities, including nuclear".
South Korea, for its part, is eager to reassure its increasingly nervous public about the US commitment to so-called extended deterrence, in which US military assets, including nuclear weapons, serve to prevent attacks on allies.