Inter-Korean tensions escalated on Saturday as the Democratic People's Republic of Korea (DPRK) test- fired ballistic and cruise missiles to the eastern coast of the Korean Peninsula.

The DPRK test-fired three ship-to-ship missiles for about an hour from 4:25 p.m. local time (0725 GMT) off its northeast coast, the South Korean military was quoted by local media as saying.

Those were believed to be "KN-01" cruise missiles. Top DPRK leader Kim Jong Un oversaw firing of the missiles on Feb. 6.

The test-firing came after DPRK issued warnings for two consecutive days of "direct aimed strikes without any prior notice " against any South Korean navy patrol ships, which Pyongyang accused of violating the disputed western sea border.

The DPRK's southwestern front command issued the warning Friday, saying 17 South Korean naval boats intruded into its territorial waters in the past seven days.

The warning was sent via the western military communication line to South Korea's presidential office Cheong Wa Dae, and the DPRK command sent another "threatening" notice to Seoul on Saturday.

The DPRK-claimed incursion was a repeat of long-standing inter- Korean dispute over the western maritime boundary. Seoul sees the Northern Limit Line (NLL), which was drawn by U.S.-led forces after the Korean War ended in 1953, as an official demarcation line, while Pyongyang has demanded the border farther south.

As a response, the South Korean military said on Friday that it will "sternly" retaliate against any DPRK provocations.

More alarming was the DPRK's announcement of its success in firing a submarine-launched ballistic missile (SLBM), which could mean more powerful nuclear threats to the U.S. mainland as well as South Korea.

The DPRK's official KCNA news agency reported on Saturday that it has test-fired a ballistic missile from a submarine, which was overseen by top leader Kim.

It was believed to an initial phase of the SLBM, or an underwater launch of a ballistic missile "soaring into the sky," but it would be a real threat of nuclear strike against the United States and South Korea in the future.

The fully-developed SLBM would mean a DPRK submarine secretly sailing underwater to the U.S. waters and firing ballistic missiles to strike major military facilities and the mainland.

Pyongyang has test-fired long-range ballistic missiles, including intercontinental ballistic missiles (ICBM), five times, and the latest was the launch of the "Unha-3" rocket to put a satellite into orbit in December 2012.

It resulted in a widespread belief that Pyongyang's long-range missiles are capable of striking the western part of the U.S. mainland.

While the ICBMs can be detected before launches by surveillance satellites and early warning aircraft, the SLBM is very hard to detect and preemptively strike. It could trigger an overhaul in South Korea's missile defense system.

The DPRK's third nuclear test in February 2013 raised concerns for its miniaturizing nuclear devices. If Pyongyang succeeds in producing a nuclear warhead that can be mounted onto the SLBM, its nuclear operational capability would become a real threat.