North Korea conducted another ejection test of a ballistic missile from a submarine last month, but it was apparently unsuccessful, a South Korean military official said Wednesday.

"We have determined that North Korea conducted a submarine-launched ballistic missile ejection test last month," the official said, requesting anonymity.

It seems that the test ended up a failure, he said, adding that it is expected to take "considerable time" for the secretive communist country to master the SLBM-related technology.

Earlier in the day, the Washington Free Beacon reported that North Korea succeeded in the underwater test of a KN-11 missile near the eastern port of Sinpo on Dec. 21, citing unidentified U.S. defense officials.

It followed a Nov. 28 test in which a KN-11 missile failed to leave the water. The failure damaged the North's first missile submarine, identified by officials as the Gorae, which means whale in Korean, the report said.

The report provided no further details, not even if the missile's engine ignited after the ejection or whether the missile took flight.

But it quoted an unidentified official as saying that based on the latest successful ejection test, North Korea could be as little as a year away from deploying a submarine armed with a nuclear-tipped missile.

However, the South Korean military official played down the North's SLBM capability, arguing that Pyongyang would need more time before deploying SLBMs, although it is continuing relevant tests.

The North remains silent about the reports of its SLBM test and the Pentagon refused to confirm it.

Meanwhile, the North Korea-monitoring website, 38 North, also said that the reports of last month's SLBM test appear to be true and supported by new commercial satellite imagery of the Sinpo South Shipyard. Despite November's failed test, the North is continuing to actively pursue its SLBM development program, it said.

Satellite imagery shows activity at the secure submarine area of the shipyard that is similar to the level of activity that has been previously seen prior to an SLBM test in May, 38 North said.

In addition, 38 North said that the structure used to support a rocket engine, missile or launch tube, which is usually present either immediately prior to or shortly after a test is conducted, is in place. Also, the Sinpo-class submarine docked at the secure boat basin was seen with netting concealing ongoing work.

"North Korea's development of an SLBM and associated ballistic missile submarine has the potential to present a significant threat in the future. However, the development of an operational system will be an expensive, time-consuming endeavor with no guarantee of success," 38 North said.

In May, North Korea claimed it successfully carried out an SLBM test underwater, renewing tensions on the Korean Peninsula. The North's SLBM capability, if fully developed, would pose a serious threat because its mobile nature would make it very difficult to detect preparations for a launch.

But experts said it was believed not to be a full-scale test, but an "ejection" test conducted in an early stage of developing SLBM capabilities. U.S. officials said the North also exaggerated progress and is many years away from developing an SLBM.

Under U.N. resolutions, the North is banned from any ballistic missile activity. (Yonhap)