North Korea's military is conducting a wintertime exercise, as South Korea and the United States are closely monitoring related moves, Seoul's defense authorities said Tuesday.

"We believe that wintertime drills by North Korea's military are under way," Col. Kim Jun-rak, the spokesperson of the Joint Chiefs of Staff (JCS), told reporters.

The North usually kicks off a regular military training in December, which continues through early spring often involving artillery firing drills.

Kim declined to comment on a report of a submarine shipyard located in the city of Sinpo on North Korea's east coast, where the country test-launched what it claimed to be a new type of submarine-launched ballistic missile (SLBM) in October.

"The intelligence authorities of South Korea and the United States have been closely cooperating and keeping close tabs on related movements," he said. "As of now, there is nothing to comment further."

38 North, a U.S. website monitoring North Korea, reported last week that the reclusive country has placed a Sinpo-class submarine at a dry dock of the shipyard, possibly for maintenance or repair. (Yonhap)