North Korea has fired two ballistic missiles into the sea, according to US and South Korean officials.

A South Korean defence ministry spokesman said the missiles were fired from Sukchon in the country's southwest at 5.55am local time on Thursday.

They reportedly flew 800km into the East Sea, also known as the Sea of Japan.

One US defence official, speaking on condition of anonymity, said the US also tracked the launch of a second ballistic missile from the same region.

The launch is the latest in a series of nuclear and rocket tests carried out by the isolated country this year which have prompted the UN and United States to impose their toughest sanctions on North Korea to date.

Neither US or South Korean officials could confirm the type of missile used in this latest launch, but the 800km distance it covered indicates it had a longer range than most of the short-range missiles in the North's arsenal.

That would make it the first test of a medium range missile, capable of reaching Japan, since 2014.

Monitoring the situation

In a statement, the US State Department said it was closely monitoring the situation.

Japan, meanwhile, said it would take "all necessary measures" to protect itself against North Korean hostility.

Prime Minister Shinzo Abe told parliament: "Japan strongly demands North Korea to exercise self-restraint and will take all necessary measures, such as warning and surveillance activity, to be able to respond to any situations."

North Korea conducted its fourth nuclear test on 6 January, prompting international condemnation.

A month later it launched a long-range rocket that the US and its allies said employed banned ballistic missile technology.

On March 10 it fired two short-range missiles into the East Sea in response to joint military exercises conducted by the US and South Korea.

It has since vowed to carry out further ballistic missile launches and a nuclear warhead test.

American sentence to hard labour

On Wednesday North Korea sentenced an American student to 15 years hard labour after he admitted stealing propaganda material on a visit to the country in January.

In a statement made before his trial, 21-year-old Otto Warmbier told reporters in Pyongyang he wanted the banner "as a trophy" for the mother of a friend.

White House spokesman Josh Earnest condemned the sentencing, saying North Korea was using US citizens as "pawns to pursue a political agenda".

He demanded the immediate release of Mr Warmbier, a University of Virginia undergraduate student.