North Korea fired seven surface-to-air missiles into the sea off its east coast on Thursday in an operation supervised by leader Kim Jong-Un at a time of heightened military tensions, South Korea's Defence Ministry said Friday.

A ministry spokesman said Kim was understood to have been on hand when the rockets were fired early Thursday evening from a site near the eastern town of Sondok.

The exercise came at a time of elevated tensions with North Korea's condemnation of annual US-South Korea military drills that Pyongyang views as rehearsals for invasion.

"We see this as another show of force by the North related to the exercises," the spokesman told AFP.

One of the joint drills, Key Resolve, wound up Friday, but the other, Foal Eagle, is set to continue until April 24.

The annual exercises are always a particularly testing time for relations between the two Koreas, who remain technically at war because the 1950-53 Korean conflict ended with a ceasefire rather than a peace treaty.

North Korea signalled its displeasure when this year's drills began by firing two short-range Scud missiles into the sea off its east coast.

UN resolutions ban any ballistic missile test by North Korea.

The defence ministry spokesman acknowledged that Thursday's surface-to-air missile tests were not a violation of the resolutions, but added that Pyongyang had failed to announce the firing in advance as a caution to shipping.

- 'Low-intensity' demonstration

"This is a pretty low-intensity show of force, timed to coincide with the wrap-up of the Key Resolve exercise," said Yang Moo-Jin of the University of North Korean Studies in Seoul.

Missile tests have long been a preferred North Korean method of expressing displeasure with what it views as confrontational behaviour by the South and its allies, though Seoul and Washington insist the exercises are defence-based in nature.

Although there is no doubt that North Korea has an extremely active ballistic missile development programme, expert opinion is split on how much progress it has made.

In 2012, Pyongyang demonstrated its rocket capabilities by sending a satellite into orbit, but it has yet to conduct a test that would show it had mastered the re-entry technology required for an inter-continental ballistic missile (ICBM).

The other key question is how close the North is to being able to miniaturise a nuclear device that could be fitted on the tip of a missile.

In a white paper published earlier this month, the South Korean defence ministry said the North had already taken its miniaturisation technology to a "significant" level.

A US Defense Intelligence Agency report leaked in 2013 reached the same conclusion, although US officials at the time said it did not represent a consensus view of all the country's spy agencies.

North Korea has carried out three nuclear tests -- in 2006, 2009 and 2013.

In January, the North offered a moratorium on further tests if this year's joint military drills were cancelled -- a proposal rejected by Washington as an "implicit threat" to carry out a fourth atomic detonation.