North Korea has fired two short-range ballistic missile into the Sea of Japan, South Korea's military has said.

The two missiles were fired at around 5:20 am on Wednesday from North Hwanghae Province and hit waters northeast of the port city of Wonsan off the country's east coast, according to The Joint Chief of Staff (JCS), Yonhap news agency reports.

Sources told the South Korean agency the missiles flew around 500 kilometers.

The JCS said: "The military is keeping close tabs on the situation and prepared to deal with any North Korean provocations."

North Korean leader Kim Jong-un, talking with scientists and technicians involved in the research of nuclear weapons, at an undisclosed location (Getty Images)

The announcement comes after Kim Jong-un said on Tuesday North Korea now possesses nuclear warheads small enough to fit on ballistic missiles.

The kingdom made the claim by publishing photos on the front page of its ruling party newspaper that show the country's leader standing beside what it said was miniaturised nuclear warhead during a meeting with his top scientists.

The Rodong Sinmun newspaper photos showed Kim and the scientists standing by what outside analysts say appears to be the model warhead - a small, silverish globe presented on a low table in a hangar with a ballistic missile or a model ballistic missile in the background.

This is the second time North Korea have launched missiles into the Sea of Japan in just one week.

On 3 March the country fired short-range missiles into the Sea of Japan, just hours after the UN Security Council voted in favour of sanctions against the regime.

North Korea launch short range projectiles Show all 6 1 /6 North Korea launch short range projectiles North Korea launch short range projectiles A man watches a TV news program showing a file footage of the missile launch conducted by North Korea, at Seoul Railway Station in Seoul, South Korea, AP North Korea launch short range projectiles North Korean leader Kim Jong-un speaking during a ceremony for the scientists, technicians, workers and officials who worked on the recent successful launch of a satellite EPA North Korea launch short range projectiles People watch a TV news program showing a file footage of the missile launch conducted by North Korea, at Seoul Railway Station in Seoul, South Korea, AP North Korea launch short range projectiles North Korean soldiers guard the truce village of Panmunjom at the Demilitarized Zone (DMZ) which separates the two Koreas, in Panmunjom, North Korea AP North Korea launch short range projectiles People watch a TV news program showing North Korean leader Kim Jong Un, at Seoul Railway Station in Seoul, South Korea AP North Korea launch short range projectiles An undated file picture released by the Rodong Sinmun, the newspaper of the North Korean ruling Workers Party EPA

The UN passed the toughest sanctions in two decades against North Korea after it carried out a nuclear bomb test and launched a long-range missile, causing security tensions to increase.

As a result of the sanctions, all cargo going in and out of the country will be inspected, while 16 more people and 12 organisations have been blacklisted.