The South Korean government has said that North Korea could have a submarine fleet capable of launching missiles in fewer than five years. Its assessment follows North Korea's publication of pictures showing its leader Kim Jong-un observing a missile test launch from a submarine last week.

According to BBC News, South Korea says the test is "very serious" but that the technology is still at an "elementary stage".

North Korea has said it is developing nuclear war-heads.

Following the launch, Kim said his country now possessed a "world-level strategic weapon capable of striking and wiping out in any waters the hostile forces infringing upon [North Korea's] sovereignty and dignity", state media reported.

In April, a U.S. think-tank claimed satellite pictures taken earlier this year suggest North Korea may have restarted one of its nuclear reactors.

The Institute for Science and International Security says images of the Yongbyon plant show patterns of melting snow indicating new activity.

Yongbyon's reactor was shut down in 2007 but was restarted in 2013.