North Korea reportedly fired three ballistic missiles into the Sea of Japan, during what's believed to be yet another upset, but this time over a new missile defence system the US is deploying in South Korea.

The missiles were launched from the western city of Hwangju. The first two missiles were believed to be short-range Scud missiles while the third was likely a mid-range Rodong, according to US officials.

While recent missile launches are generally seen as efforts by the North to improve its missile capability, the recent launches were intended as a show of force, Reuters reports.

"This smells political rather than technical to me," said Melissa Hanham, a senior research associate at the Middlebury Institute of International Studies at Monterey, California.

The trio of tests took place early morning on Tuesday 19 July, six days after South Korea announced it will deploy the US Thaad anti-missile system southeast of Seoul to counter Pyongang's threats.

The North has warned it would retaliate against the new system with a "physical response", while China has warned that the move will destabilise security in the region.

The three launched missiles travelled between 310 (500km) and 370 miles (600km), said a statement from South Korea's Joint Chiefs of Staff. That distance is "far enough to strike all of South Korea including Busan," an important port city, the statement noted.

North Korea is barred by UN sanctions from any nuclear or ballistic missile technology test. But the hermit nation conducted its fourth nuclear test in January (2016) but has launched several missiles.

Seoul has warned that recent hectic activity tracked at North Korea's Punggye-ri nuclear test site indicates Pyongyang is preparing to conduct a fifth nuclear test any day.