North Korea has fired two medium-range ballistic missiles into the sea, US and South Korean officials have said.

South Korea's military said the first missile was launched from Sukchon in the country's southwest at 5:55am on Friday (20:55 GMT on Thursday) and flew 800km before crashing off into the East Sea, also known as Sea of Japan.

The second, fired about 20 minutes later, disappeared off radar early into its flight.

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Both missiles are believed to be medium-range Rodong missiles fired from road-mobile launch vehicles.

If confirmed, it would mark North Korea's first test of a medium-range missile, capable of reaching Japan, since 2014 - the Rodong missile has an estimated maximum range of around 1,300km.

'Defiant gesture'

The launches came amid heightened tensions on the Korean peninsula after the North rejected UN Security Council sanctions imposed earlier in March in response to a nuclear test conducted in January.

New US sanctions on Pyongyang were also issued on Wednesday aiming to expand North Korea's blockade by blacklisting individuals and entities that deal with the North's economy.

Jeff Kingston, a professor of Asian Studies at Temple University in Tokyo, Japan, said the test firing was "clearly" a "defiant gesture by Pyongyang.

"[They] say, 'well we are not going to back down and we are going to continue with our quest to become a nuclear weapons power,'" Kingston told Al Jazeera.

"What we've discovered over the last 20 years is that there's not really any combination of sticks and carrots that seems to deter them from that quest, so it looks like we're still at an impasse and it's really hard to see a bright road ahead."

International reaction

The US State Department said in a statement it was closely monitoring the situation and urged North Korea to refrain from any actions that could "further raise tensions".

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South Korea also condemned the North's move, calling it a direct challenge to the UN Security Council and the international community, while Japan lodged a protest through its embassy in Beijing.

"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," Prime Minister Shinzo Abe told parliament.

Al Jazeera's Rob McBride, reporting from Hong Kong, said the move was also likely to get a strong response from Beijing, North Korea's main diplomatic ally.

"There is increasing alarm in China in what they see as the growing instability on the Korean peninsula and also frustration of their waning influence over North Korea in trying to pull them back from carrying out these kinds of tests," McBride said.

"For the international community, if they were wondering whether the latest sanctions imposed on North Korea were going to get them to change their ways, now they seem to have their answer," he added.

Military drills

North Korea often fires missiles at periods of tension on the Korean peninsula or when it comes under pressure to curb its defiance and abandon its weapons programmes.

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Last week, Pyongyang fired two short-range missiles into the sea off its east coast and its leader Kim Jong-un ordered more nuclear weapons tests and missile tests to improve attack capability.

The North has also reacted angrily to annual joint military drills by US and South Korean troops that began on March 7, calling the exercises "nuclear war moves" and threatening to wipe out its enemies.