Nuclear-armed North Korea launched four ballistic missiles on Monday in another challenge to President Donald Trump, with three landing provocatively close to America's ally Japan.

Seoul and Washington began annual joint military exercises last week that always infuriate Pyongyang, with the North’s military warning of “merciless nuclear counter-action”.

Under leader Kim Jong-Un, Pyongyang has ambitions to develop an Intercontinental Ballistic Missile (ICBM) capable of reaching the US mainland -- which Trump has vowed will not happen.

Seoul said four missiles were fired from Tongchang County, North Pyongan province into the East Sea -- its name for the Sea of Japan -- and that South Korea and the US were “closely analyzing” tracking data for further details.

The missiles travelled around 1,000 kilometers (620 miles) and reached an altitude of 260 kilometers, said a spokesman for South Korea's Joint Chiefs of Staff, adding they were unlikely to be ICBMs.

Japanese Prime Minister Shinzo Abe said three of the North Korean missiles came down in Tokyo’s Exclusive Economic Zone (EEZ) -- waters extending 200 nautical miles (370 kilometers) from its coast.

“This clearly shows North Korea has entered a new stage of threat,” Abe told Parliament.

The North’s repeated launches “clearly violate UN Security Council resolutions. We can never tolerate this,” he continued.

Tokyo’s chief government spokesman Yoshihide Suga added that Japan was considering calling for an emergency Security Council meeting.

Pyongyang carried out two atomic tests last year and a series of missile launches, but Monday was only the second time its devices had entered Japan’s EEZ.

In Washington, the State Department strongly condemned the launches, saying the US was ready to “use the full range of capabilities at our disposal against this growing threat”.

“We remain prepared -- and will continue to take steps to increase our readiness -- to defend ourselves and our allies from attack,” acting spokesman Mark Toner said.

After an emergency meeting of South Korea’s National Security Council, Acting President Hwang Kyo-Ahn called the North’s nuclear and missile provocations “immediate and real threats” to his country.

“Considering the North Korean leadership’s brutality and recklessness shown through the murder of Kim Jong-Nam, the results of the North having a nuclear weapon in its hands will be gruesome beyond imagination,” he said.

Seoul has blamed Pyongyang for the killing of the half-brother of the North’s leader by two women using VX nerve agent at Kuala Lumpur's international airport last month.

Hwang called for “swift deployment” of a US missile defense system, THAAD, a proposal which has infuriated neighboring China, the North’s key diplomatic protector and main provider of trade and aid.

Monday’s launch came ahead of a trip to Japan, China and South Korea by new US Secretary of State Rex Tillerson this month, pointed out Dongguk University professor Kim Yong-Hyun.

Pyongyang was “trying to send a message early on in Trump’s term that North Korea will not be dragged around by his administration”, he said.