North Korea said Monday it had test-fired two strategic cruise missiles from a submarine, as South Korea and the United States kicked off their largest joint military exercises in five years.

Nuclear-armed Pyongyang said the test verified its "nuclear war deterrence means in different spaces" as it slammed the drills -- known as Freedom Shield -- which will run for 10 days from Monday as part of the allies' drive to counter North Korea's growing threats.

"The two strategic cruise missiles precisely hit the preset target on the East Sea of Korea," the official Korean Central News Agency (KCNA) said.

North Korea is not technically banned from firing cruise missiles under current UN sanctions -- although tests relating to its nuclear arsenal are not allowed.

The KCNA report said the test was linked to the United States and South Korea "getting evermore undisguised in their anti-DPRK military manoeuvres", referring to the North by its official name.

The South Korean military said it had detected the launch of at least one unidentified missile from a North Korean submarine Sunday morning.

Photos and video released by North Korean state media showed the submarine, the "8.24 Yongung", and a missile flying into the sky from the water, trailing white smoke and flames.

Analysts said "huge doubts" remain about how advanced the North's submarine programme is.

Park Won-gon, a professor at Ewha University in Seoul, said the state media images suggested the missile was fired from above the water.

"Then there is no point in shooting from a submarine because there is no stealth," Park told AFP.

"North Korea says the weapons are deployed, but whether we believe it with credibility is another matter."

Defensive drill

The Freedom Shield drills "involve wartime procedures to repel potential North Korean attacks and conduct a stabilisation campaign in the North", the South Korean military has said.

It emphasised that the exercise was a "defensive one based on a combined operational plan".

But North Korea views all such exercises as rehearsals for invasion and has repeatedly warned it would take "overwhelming" action in response.

"North Korea has been speaking in missiles against joint drills," said Go Myong-hyun, a researcher at the Asan Institute for Policy Studies in Seoul.

"It wants to emphasise that the reason for developing missiles is for self-defence purposes."

The foreign ministry in Pyongyang also released a statement Monday slamming the United States over what it called "the US vicious 'human rights' racket", after Washington said it would hold a UN meeting this week on abuses in North Korea.

More to come

Last year, North Korea declared itself an "irreversible" nuclear power and fired a record-breaking number of missiles.

Leader Kim Jong Un last week ordered his military to intensify 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.

Although the official policy of both countries towards North Korea -- that Kim must give up his nukes and return to the table for talks -- has not changed, experts said there had been a practical shift.

The United States has "effectively acknowledged that North Korea will never give up its nuclear programme", An Chan-il, a defector turned researcher who runs the World Institute for North Korea Studies, told AFP.

The Freedom Shield drill is the first since that happened, meaning it "will be very different -- both qualitatively and quantitatively -- from previous joint exercises that took place in recent years", he added.

North Korea will likely use this "as an excuse" to double down on its banned weapons programmes, said Chun In-bum, a retired South Korean army general.

"More missile launches with variations in style and scope should be expected with even a nuclear test. More acts of intimidation from North Korea should not come as a surprise."