U.S. intelligence agencies believe North Korea is building a submarine capable of launching ballistic missiles, potentially increasing the threat posed by the nuclear-armed rogue state.

A missile launch tube on a North Korean submarine was observed recently by U.S. intelligence agencies and is raising new concerns about the missile and nuclear threat from the communist regime in Pyongyang, according to two defense officials familiar with reports of the development.

Pentagon spokesman Lt. Col. Jeffrey Pool declined to comment on the North Korean missile submarine program. "We do not comment on intelligence matters, but we do urge North Korea to become more transparent in their defense sector in order to reduce tensions in the region," he said in a statement.

Details of the missile submarine remain closely held within the government.

Disclosure of the missile submarine work followed a recent report from North Korea showing photographs of dictator Kim Jong Un riding in the conning tower of a rusty North Korean submarine at sea.

Military analysts expressed surprise at the intelligence on the missile submarine program and speculated the submarine could be a modification of one of North Korea’s Russian or Chinese design Romeo-class diesel submarines.

A second possibility is that the new missile submarine is a copy or modification of a Soviet-era Golf-class missile-firing submarine purchased by Pyongyang—ostensibly for scrap metal—in the mid-1990s.

North Korea’s current submarine force includes around 70 submarines. Most of the vessels are older diesel submarines, including 22 Romeo-class or Chinese design Type-031 subs—the type of submarine Kim was photographed on in June.

The majority of the North’s submarines, more than two dozen, are small, Sango-class mini-submarines used for covert commando operations to infiltrate South Korea during a conflict. The North Koreans also have four 1940s-design Whiskey-class submarines from Russia.

North Korea’s use of submarine warfare was evident as recently as 2010, when it carried out a torpedo attack on South Korea’s Choenan costal warship, killing 46 sailors. A North Korean mini-submarine is suspected in that attack. North Korea denied it was behind the strike.

Additionally, U.S. intelligence agencies believe North Korea already has submarine-launched ballistic missiles (SLBMs). The North covertly obtained several SS-N-6 SLBMs from Russia. The missile has been adapted into North Korea’s new intermediate-range missiles.

Until the recent discovery of the modified submarine, there were no intelligence signs North Korea is interested in developing its own submarine-launched missiles.

Pyongyang is one of the largest producers and exporters of ballistic missiles in the world, and makes several types of short-range Scuds, medium-range Nodongs, Musudan intermediate range missiles, and long-range Taepodongs.

Its missile arsenal is estimated to include between 600 and 1,000 missiles.

If the modified submarine is based on the Golf, it is likely the technology or actual missile tubes were derived from the decommissioned submarines obtained in the mid-1990s.

The authoritative Jane’s’ Fighting Ships revealed in May 1994 that North Korea purchased 40 decommissioned submarines from Russia, including several Golf-class and Romeo-class submarines.

Former British Navy Capt. Richard Sharpe, stated in the foreword to the 1994 publication that the Golf submarine sale raised concerns about their missile capabilities. It is "possible that the missile tubes may be adaptable for other weapons," including ballistic missiles, he stated.

Rick Fisher, a military analyst, said he suspects one of the Golf-class submarines obtained by the North Koreans included one or more SS-N-6 missiles in its launch tubes as part of the deal.

North Korea then may have used the past 20 years to reverse-engineer a version of the Golf, which was designed to carry up to three ballistic missiles, Fisher said.

"In North Korean service, a Golf-size ballistic missile-carrying submarine could potentially carry two Musudan-size liquid-fueled missiles or a larger number of long-range cruise missiles," he said.

If the missile submarine development is confirmed, it would mean North Korea could use one of its submarine missiles to attack Anchorage from waters near Russia’s Sakhalin Island. Another potential target for the North Korean missile submarine could be U.S. forces on Okinawa, Philippines, or Guam from the Yellow Sea. The missile could have a range of between 1,500 and 2,500 miles.

Eric Wertheim, an analyst at the U.S. Naval Institute, said he is skeptical North Korea could develop missile firing submarines, either indigenously or by copying or refurbishing a Golf.

"There are very many hurdles that would need to be overcome for North Korea to develop an SLBM and turn a diesel submarine into an operational platform able to launch guided SLBMs," Wertheim said in an email.

"If not a Golf class, launch of SLBMs would probably require another large class of submarine and most recent North Korean submarine production seems to have focused on smaller subs or less than 1,000 tons," he added.

A former intelligence analyst also said North Korea’s refurbishing of the Golf subs would be very difficult, and that the North Koreans would need satellite navigation for the missile.

South Korea press reports, however, have indicated recently that China is sharing its Beidou satellite navigation technology with North Korea.

A 2009 State Department cable made public by Wikileaks revealed that North Korea developed a new land-mobile intermediate-range missile based on the SS-N-6 SLBM.

The missile was deployed on Russia’s Golf submarines, indicating that the Golf missile tubes could be used or copied for the new North Korean missile submarine or as launch tubes on a converted submarine.

A modified SS-N-6 engine was used to build what is now called North Korea’s road-mobile Musudan missiles.

"Development of the Musudan with this more advanced propulsion technology allows North Korea to build even longer-range missiles—or shorter range missiles with greater payload capacity—than would be possible using Scud-type technology," the cable on North Korean missile development stated.

A second 2011 cable revealed that North Korea shared the SS-N-6 missile technology with Iran for use in its Safir missile. The cable said "the second-stage of the Safir utilizes steering engines that are almost certainly derived from the Soviet-era SS-N-6 (Soviet designation: R-27 or RSM- 25) submarine-launched ballistic missile."

"U.S. information was clear that Iran purchased these [missiles] from North Korea and that its technology was far more advanced than SCUD technology," a third cable from 2007 said.

That cable quoted Russian officials, in talks with U.S. officials, denying any SS-N-6 missiles were sold to North Korea, claiming all were destroyed as part of the 1987 Intermediate-range Nuclear Forces treaty, which the State Department recently said was violated by Moscow through development of a new cruise missile.

Additionally, the Russians in 2007 said North Korea and Iran could not master the technology of the SS-N-6. "The Russians argued that the technology was customized for underwater use; and that the missile would not work without the water pressure at specific depths," the cable said.

Fisher said another possible source for North Korea’s submarine-launched missile program is China, which has produced a version of the Golf known as the Type-031 and which was used until 2013 as an SLBM test platform.

"Given China's habit of assisting North Korea to obtain earlier generation strategic weapons, the chances are greater that China would have aided North Korea's rebuilding of the scrap Soviet Golf class submarines," Fisher said. "Chinese assistance would have been crucial for North Korea to rebuild the pressure hull, missile tubes and to equip the sub with modern command and control systems."

Fisher noted covert Chinese missile assistance in the past to North Korea’s KN-08 long-range missile, specifically the transfer of Chinese-made transporter-erector launchers.

"The emergence of a North Korean ballistic missile carrying submarine raises many questions for U.S. strategy and force levels in Asia," Fisher said. "Should it now be considered a great error for the Obama administration to have retired and dismantled U.S. nuclear-armed cruise missiles for U.S. submarines in 2010?"

A missile-firing North Korean submarine also will require increased U.S. and Japanese anti-submarine warfare forces, he said. "Both will have to consider an additional number of submarines to check North Korea," Fisher said.

Another concern is that North Korea will share or export missile-firing submarines in the future to Iran. North Korea cooperated with Iran on submarine training in the past.