North Korea has fired three ballistic missiles into the sea off its east coast, South Korea's military said, just over a week after issuing threats to respond to the planned deployment of a US anti-missile system in the South.

Key points: North Korean launches demonstrate its missile can reach the South's port city of Busan

North Korean launches demonstrate its missile can reach the South's port city of Busan June saw the North's first successes in mid-range Musudan missile launches

June saw the North's first successes in mid-range Musudan missile launches Launch assumed to be in response to THAAD anti-missile system to be deployed in the South

US Strategic Command said in a statement it detected the launches of what it believed were two Scud tactical ballistic missiles and a No Dong intermediate-range ballistic missile.

The missiles, launched early on Tuesday from the western city of Hwangju, flew far enough to reach the very edge of South Korea, including its port city in the south.

"The ballistic missiles flight went from 500 kilometres to 600 kilometres, which is a distance far enough to strike all of South Korea including Busan," the Joint Chiefs of Staff said in a press statement.

North Korea has test-fired a series of ballistic missiles in recent months, including an intermediate-range missile in June and a submarine-launched missile this month.

After four failed mid-range Musudan missile launches up to May this year, a successful test in June marked a major step forward for a weapons program that ultimately aspires to develop a proven nuclear strike capability against the US mainland.

Tuesday's launch came days after South Korea and the United States announced a final decision this month to deploy the Terminal High Altitude Area Defence (THAAD) anti-missile system in the South to counter threats from the North.

North Korea's military has threatened to retaliate against the deployment of the system with a "physical response" once its location and time of installation were decided.

China has also sharply criticised the decision as a move that will destabilise the security balance in the region.

THAAD is a truck-mounted system that can intercept an enemy missile launch. ( AFP, file photo )

AFP/Reuters