North Korea has tried to enhance its submarines to make them capable of firing ballistic missiles as it seeks to boost its ability to launch surprise attacks, a South Korean official said.

North Korea has been attempting to modify its Soviet-era submarines so they can be used to launch “SSN6” missiles, South Korean Defense Ministry spokesman Kim Min Seok said today at a briefing. The North hasn’t yet succeeded in upgrading the subs to make them capable of firing missiles, he said.

“North Korea is developing an array of weapons that can be used in surprise attacks,” Kim said. “It is ratcheting up its threat against South Korea with those asymmetrical capabilities because it is facing challenges with conventional capabilities.”

North Korea has 1.2 million troops and about 70 submarines, while the South has 10 of the vessels, according to a 2012 South Korean defense white paper. South Korea accused the North of torpedoing one of its warships in 2010, resulting in the deaths of 46 sailors, a charge the North denies.

North Korea is banned from testing or developing ballistic missiles under sanctions imposed by the United Nations over the three nuclear tests its held since 2006. The two countries have faced each other over one of the world’s most heavily fortified borders since their 1950-53 war ended in a truce.

North Korea routinely fires missiles in tests or drills. Leader Kim Jong Un personally oversaw the launch of “ultra- precision” guided missiles in June.