South Korea on Tuesday warned North korea it was playing a dangerous game with a series of missile, rocket and artillery tests that appear to be inching ever closer to their joint border.

In the latest incident on Monday, the North fired 100 shells into the sea from multiple rocket launchers in a live-fire drill close to the eastern maritime boundary.

"Some civilian tourists at the east coast even saw the water splash after the shells fell in the sea, which is very threatening to our country," said ministry spokesman Kim Min-Seok.

North Korea regularly fires off missiles and rockets, but the frequency of the recent tests -- six in less than three weeks -- is unusual.

They have included artillery shells, short-range rockets and Scud missiles with a range of 500 kilometres (310 miles) -- all fired into the Sea of Japan (East Sea) from various locations.

Most have been personally monitored on-site by North Korean leader Kim Jong-Un.

Kim Min-Seok stressed that a stray missile or shell risked triggering a serious confrontation.

"Our stance is clear," he told a press briefing.

"We will retaliate without hesitation if the North sends any of its missiles or shells to the south of the border."

UN resolutions bar North Korea from conducting any launches using ballistic missile technology, but the response to the recent tests has so far been limited to verbal protests from Seoul, Tokyo and Washington.

Analysts see numerous possible motives behind the tests: pique over Chinese President Xi Jinping's recent visit to South Korea, anger over joint South Korean-US military drills, and a general effort at some attention-seeking muscle flexing.

While guiding Monday's live-fire exercise, Kim Jong-Un was quoted by the North's official KCNA news agency as saying hostile forces were becoming more blatant in their moves to "isolate and stifle" North Korea.

Later the same day, US President Barack Obama and Xi Jinping had a telephone call during which they discussed the need to ensure North Korea complies with demands to dismantle its nuclear program.

The North Korean tests have coincided with various peace overtures to Seoul, including a proposal to halt all provocative military activity.

Officials from both sides are due to hold rare talks on Thursday to discuss North Korea's participation in the upcoming Asian Games in the South Korean port city of Incheon.

South Korean President Park Geun-Hye has accused Pyongyang of adopting a "two-faced attitude" by proposing a lowering of tensions while continuing its missile tests.