North Korea has launched a new submarine capable of firing ballistic missiles, military and government sources in Seoul said Sunday, raising further concerns over the North's evolving missile and nuclear threats.

The communist country "imported a Soviet-era Golf-class diesel submarine and modified it," a government source said on condition of anonymity. The Soviet vessel was built in 1958 and decommissioned in 1990.

"The new submarine is 67 meters long with a beam of 6.6 meters, and has a dived displacement in the 3,000-ton range," the source said. 

Despite a series of reports on Pyongyang's possible development of a new submarine, Seoul military officers have said Pyongyang has not yet acquired technology to deploy submarine-launched ballistic missiles (SLBMs).

The Russian 3,500-ton Golf II class submarine carries the R-21 SLBM, a single-stage, liquid-propellant missile with a 1,180-kilogram warhead and a maximum range of 1,420 kilometers.

"The North's new vessel is what the website 38 North reported last month as 'an unidentified submarine' moored in a boat basin at the Sinpo South Shipyard, citing its review of satellite imagery," the source said.

The website is run by the U.S. Korea Institute at the Johns Hopkins School of Advanced International Studies. 

As the North's primary submarine manufacturing facility in South Hamgyong Province on the east coast, the Sinpo shipyard is home to the headquarters of its Maritime Research Institute of the Academy of National Defense Sciences.

In a move to mount a missile tube on the new vessel, the communist country has carried out dozens of tests both on the ground and at sea, another source said. 

"According to the analysis of satellite imagery revealed by 38 North, a ground test facility for the SLBM launch has been up and running at the Sinpo shipyard," he said, adding a dozen more tests would be required to perfect the technology.

His comments are in line with what arms expert Joseph S.Bermudez Jr. said in his report this month that North Korea has built "a new test stand" at Sinpo to research and develop SLBMs. He said that the installation has a 35-by-30-meter concrete pad with an approximately 12-meter-high test stand. 

"It would take one or two years before the North completes the test for the vertical launch of missiles from the sea," said a military source in Seoul, expressing security concerns as Pyongyang has also been working on miniaturizing nuclear warheads for its missiles. 

The belligerent regime has more submarines than the South, albeit being equipped with outdated weapons. It is believed to have some 70 submarines including some 20 1,800-ton Romeo-class submarines.

As part of efforts to bolster its anti-submarine capabilities in the wake of the North's deadly attack of its warship of Cheonan in 2010, South Korea is planning to put six 3,000-ton ballistic missile submarines into operation starting 2027. The sinking killed

46 South Korean sailors, while Pyongyang has denied any involvement. (Yonhap)