A local official on Jeju Island said Monday that a US naval ship carried out an underwater survey off the South Korean military base there last month, raising suspicions that it may be part of preparation for the deployment of US strategic assets.

"The US Navy vessel Henson was spotted conducting an extensive survey of the sea off the base late into the night from Aug. 17-29," the official at the community center of Gangjeong Village, home to the base, said.

The oceanographic survey ship went farther from the base, formerly called the Jeju Civilian-Military Complex Port, on the last two days of its investigation, according to the official. 

Two boats unloaded from the ship are known to have examined the undersea topography to survey the sea route.

"It's doubtful whether the survey was preparatory to a full deployment of the US Navy's core strategic assets here," the official said.

The community office has requested the presidential office and defense ministry clarify the purpose of the survey, according to the official.

The naval base denied the claim.

"The survey was a regular Korea-US joint environmental probe in order to protect our naval and fishing boats from possible enemy submarine and mine attacks."

Such surveys are conducted at other major ports as well, it said. 

Construction of the Jeju Civilian-Military Complex Port was begun in 2007 and completed in February 2016 after years of delays amid vehement opposition from villagers and activists worried about possible environmental damage.

The port can accommodate 20 warships, three submarines and two 150,000-ton cruise vessels at the same time. Cruise liners are expected to begin docking at the port in the near future.

Navy ships stationed at the base have easy access to the three seas surrounding the Korean Peninsula so they can respond more promptly to any maritime threats posed by North Korea. (Yonhap)