The Korean Central News Agency said the operation early Saturday was a "counteraction drill" in response to joint military activity by US and South Korean forces that KCNA said has escalated tensions in the region.

"A firing drill for simulated tactical nuclear attack was conducted at dawn of September 2 to warn the enemies of the actual nuclear war danger," KCNA reported.

"Two long-range strategic cruise missiles tipped with mock nuclear warheads were fired" from North Korea's west coast, into the sea to the south, it said.

South Korea's Joint Chiefs of Staff said Saturday that an unspecified number of cruise missiles were launched at around 4:00 am (1900 GMT) towards the Yellow Sea, adding the specifications of the missiles were being evaluated.

KCNA said the United States and South Korea were pursuing "confrontation hysteria" with their latest joint military drills.

North Korea has conducted a record number of weapons tests this year, and last week carried out its second failed attempt to put a spy satellite into orbit.

Seoul and Washington have ramped up defence cooperation in response, staging joint military exercises with advanced stealth jets and US strategic assets.

On Tuesday, North Korea's leader Kim Jong Un visited a training command post where he detailed future war plans, including "making simultaneous super-intense strikes" at core military posts in the South.

Relations between the two Koreas are at their lowest point in years, and diplomacy is stalled after failed attempts to discuss Pyongyang's denuclearisation.