North Korea test-fired three ballistic missiles on Tuesday, 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.

The missiles, launched early Tuesday from the western city of Hwangju, flew between 500 and 600 kilometres (311-373 miles) toward the Sea of Japan, the Joint Chiefs of Staff (JSC) said in a press statement.

The range of what were believed to be SCUD missiles could reach anywhere in South Korea, the JCS said, adding the military was keeping close tabs on the North's movement.

North Korea last week threatened to take "physical action" after Washington and Seoul announced plans deploy a sophisticated US anti-missile defence system to counter the growing menace from Pyongyang.

The Terminal High Altitude Area Defense system, or THAAD, will be deployed in Seongju county by the end of next year.

An unidentified military official said that Tuesday's launch was believed to be linked to the North's latest threats.

"SCUDs are a weapon that the North is most likely to use to hit places like Seongju", the official said, according to Yonhap news agency.

The North previously launched relatively short-ranged SCUDs in March, as it flexed its muscles in response to joint US-South Korea military drills just south of the border.

North Korea reportedly has some 600 SCUDs, developed in the former Soviet Union and modified by Pyongyang, whose ranges are estimated to be somewhere between 300 and 700 kilometres.

Tensions have soared since Pyongyang carried out its fourth nuclear test in January, followed by a series of missile launches that analysts say shows that the North is making progress toward being able to strike the US mainland.

Both South Korean and US troops stationed in the South have Patriot anti-air defence systems that can intercept enemy missiles at low altitudes.

Alongside the Patriot systems, the THAAD system is necessary to form a multi-layered anti-air defence system, South Korean military authorities say.

The announcement to deploy the THAAD system in Seongju sparked fierce protests by residents who are concerned it will ruin the town's economic main stay, melon farming, and create health and environmental hazards.