Japan’s prime minister, Shinzo Abe, has denounced North Korea’s latest missile launch as an “unforgivable” threat to regional stability after the isolated state successfully test-fired a missile from a submarine on Wednesday morning.

US Strategic Command said the missile, believed to be a KN-11, had been fired from the North Korean coastal city of Sinpo at around 5:30 am local time and flew about 500 km before landing in the Sea of Japan.

Abe noted that it was the first time a North Korean missile launched from a submarine had entered Japan’s air defence identification zone, a reference to areas that countries designate to help maintain air security.

“This poses a grave threat to Japan’s security, and is an unforgivable act that damages regional peace and stability markedly,” Abe told reporters, adding that Japan had lodged a protest with North Korea.

The foreign ministers of Japan, South Korea and China were expected to condemn the missile test – the regime’s latest show of defiance against UN security council resolutions banning it from developing ballistic missile technology - when they hold talks in Tokyo later today.

Officials in Washington said it was ready to defend Japan and South Korea, a day after North Korea threatened to launch nuclear strikes to protest the start of annual US-South Korean military exercises it regards as a rehearsal for an invasion.

“Our commitment to the defence of our allies, including the Republic of Korea and Japan, in the face of these threats, is ironclad,” said Gary Ross, a Pentagon spokesman.

“We remain prepared to defend ourselves and our allies from any attack or provocation. We call on North Korea to refrain from actions that further raise tensions in the region and focus instead on taking concrete steps toward fulfilling its commitments and international obligations.”

Wednesday’s test indicates that North Korea has made progress in achieving its aim of developing a ballistic missile capable of striking the US mainland, although experts say it does not possess a reliable nuclear missile capable of covering such long distances.

US military bases and tens of thousands of American troops in South Korea and Japan are already within reach of North Korean short- and medium-range missiles.

The North has launched numerous missiles of various types this year, including one earlier this month that landed in or near Japanese-controlled waters for the first time.

A North Korean ability to launch missiles from submarines would be an alarming development for Japan, South Korea and the US, as missiles fired from beneath the surface of the ocean are harder to detect in advance.

The 500 km distance recorded on Wednesday is greater than that achieved by previous submarine-launched ballistic missiles [SLBM], according to defence experts. The North fired two missiles from submarines earlier this year, but defence officials in Seoul believe they exploded in midair after flying fewer than 30 km.

“North Korea’s SLBM technology appears to have progressed,” a South Korean military official said.



Jeffrey Lewis at the Middlebury Institute of International Studies in California said: “I think it was probably successful. We don’t know the full range, but 500 km is either full range or a full range on a lofted trajectory. Either way, that missile works.”

North Korea has been the target of fresh UN sanctions since it conducted its fourth nuclear test in January nuclear test, which was followed by the launch of a long-range rocket.

Tensions on the Korean peninsula deepened after the recent defection of North Korea’s deputy ambassador in London to South Korea, regarded as an embarrassing loss of face for the North Korean leader, Kim Jong-un.

The South Korean president, Park Geun-hye, said the defection was a sign that “serious cracks” were emerging in the regime.

Wednesday’s launch comes after North Korea warned that the 12-day Ulchi Freedom Guardian exercises involving the US and South Korean militaries were pushing the Korean peninsula “to the brink of a war” and asked for an urgent meeting of the UN security council.

It is not unusual, however, for the regime in Pyongyang to mark the start of the annual drills with a flurry of belligerent rhetoric.

In a letter to the UN security council president, North Korea’s UN ambassador Jan Song-nam accused the US of “creating the danger of war” with the joint exercises, which began on Monday.

Pyongyang has also voiced anger at the planned deployment of a sophisticated US missile defence system in South Korea.