North Korea has warned that the US faces an "unimaginable strike at an unimaginable time" in a new threat likely to further destabilise peace in the region.
A statement from North Korea's Pan-Korean Emergency Measure Committee for Opposing Nuclear War Drills said: "The US is running amok by introducing under our nose the targets we have set as primary ones.
"The US should expect that it will face an unimaginable strike at an unimaginable time."
On Monday, the United States and South Korea launched a joint naval exercise in a fresh show of force against North Korea over its growing missile and nuclear threats, the Daily Mail reported.
Tensions over the North's weapons programmes have soared in recent months, with Pyongyang carrying out a series of missile launches and its sixth nuclear test, its most powerful yet, in defiance of multiple sets of UN sanctions.
The United States has since ramped up military drills with South Korea and Japan, its two closest allies in the region.
"The latest exercise is aimed at maintaining readiness against North Korea's naval provocation and improving capabilities to carry out joint operations," a South Korean navy spokesman told reporters.
The 10-day drills are taking place on the southern side of the de facto maritime border known as the Northern Limit Line, the spokesman added.
The US navy said earlier that the USS Ronald Reagan aircraft carrier and two US destroyers would take part.
Pyongyang habitually condemns such joint exercises and the state-run KCNA news agency on Saturday called the Ronald Reagan's participation a "reckless act of war maniacs as it only drives the tense situation on the peninsula into the point of explosion".
The exercises are the latest instance of US military hardware movements around the Korean peninsula.
The provocative statement also comes as South Korea says the top nuclear envoys from Washington, Seoul and Tokyo agreed that the allies must pursue every available avenue including dialogue and sanctions to peacefully achieve a "complete" denuclearisation of North Korea.
A Seoul meeting on Wednesday between the countries' chief negotiators for the currently stalled nuclear disarmament talks with North Korea took place shortly after US Deputy Secretary of State John Sullivan met with his South Korean and Japanese counterparts across town to discuss responses to North Korea's nuclear activities.
US envoy for North Korea policy Joseph Yun said ahead of his meeting with the South Korean and Japanese envoys that North Korea's nuclear ambitions have created a "very serious situation" that called for high-level engagement between the allies.
The so-called six-party talks that also involved China and Russia were last held in late 2008 and North Korea went on to conduct its second nuclear test in May 2009.
Earlier this week, North Korea warned that nuclear war "may break out any moment" amid claims it is developing a missile that can reach the East coast of the US.
Pyongyang said the "entire US mainland is within our firing range" and threatened "severe punishment" for America if it "dares to invade out sacred territory".