South Korean government sources are quoted as saying North Korea's military recently tested, in the Yellow Sea, new missiles designed to attack ships.
The development is viewed as a new threat to South Korea's patrol ships and destroyers operating in tense waters off the west coast off the Korean peninsula.
North Korea's missile tests are reported to have taken place once in October and, again, earlier this month near waters whose boundary is disputed by the two Koreas.
Various South Korean media outlets Wednesday quoted officials as saying North Korea appears to have modified, for an airborne launch, the decades-old Soviet SSN-2-C short-range anti-ship missile, also known as the Styx.
The reports say North Korea fired the missiles from an IL-28 bomber in the Yellow Sea.
South Korea's Defense Ministry and the military's joint chiefs of staff say they cannot comment on the reports.
Military analyst Baek Sung-joo at the state-run Korea Institute for Defense Analyses says, although information about the new weapon, is sketchy it raises fresh concerns.
Baek says the fact that North Korea can put a missile on its aircraft is something the military here needs to carefully ponder. This would give the North Koreans the capability of firing short-range missiles when encounters take place between the two Koreas near the disputed waters.
The waters off the western coastline have been the site of sporadic clashes over the years by naval forces of the rival Koreas.
North Korea no longer recognizes the Northern Limit Line as the de facto maritime boundary.
However, analyst Baek says this reported test-firing should not be seen as a provocation.
He says that while United Nations sanctions imposed on North Korea prohibit it from developing missiles, the tests are not a provocation because the point of impact of the missiles was not in South Korean territory.
South Korea's publicly-funded Yonhap news agency says the country's military, late last year, added short-range guided surface-to-air missiles to its defenses on two border islands in the western sea.
That move was made after a North Korean artillery attack in November, 2010, on Yeonpyeong island, which killed two South Korean marines and two civilians.
The two Koreas fought a three-year civil war in the early 1950s. They have technically remained at war because a peace treaty was never signed.
The United States maintains more than 28,000 military personnel in South Korea.