A nuclear-powered submarine of the People’s Liberation Army (PLA) Navy, suspected Shang-class nuclear attack submarine, has been spotted by the Taima Star ferry during a routine passage from the Matsu Islands to Keelung.

Passengers and crew of the Taima Star ferry said the submarine rose to the surface, and that it was moving north toward China.

The submarine was seen at around 1:15 p.m. on Thursday (July 4). The first sailor onboard the Taima Star ferry to catch a glimpse of the sub said he thought his eyes were playing tricks on him, reports the Liberty Times (LTN).

The nationality of the submarine was not clearly marked, but it was later confirmed as being of Chinese origin. It was moving in a northeasterly direction, heading toward China’s Zhejiang Province, says the news agency.

A Chinese submarine spotted on surface close to Taiwan-held Matsu Islands appears to be an improved Type 093G Shang-class nuclear-propelled submarines that can fire the latest vertically launched anti-ship missiles.

Maritime specialist Collin Koh, a research fellow at the S. Rajaratnam School of International Studies (RSIS), tweeted Saturday morning that: “The PLA Navy submarine spotted on surface close to Taiwan-held Matsu Islands appears to be a Type-093G Shang-class SSN, not Type-039 Yuan-class SSK based on the silhouette of its sail.”

Type 093G subs, according to military analysts, are reportedly comparable to U.S. Navy’s Los Angeles-class Flight I and the more advanced Flight III variants in terms of stealthiness.

Taiwan’s Ministry of National Defense (MND) did not address the incident in depth. Officials said only that they were fully aware of China’s activities in the area.

Later, Taiwan’s MND has reported that it monitors the country’s surrounding waters and airspace closely at all times after a Chinese submarine was allegedly spotted operating near one of Taiwan’s offshore islands earlier in the day.

Without directly confirming or denying the submarine’s alleged presence near Taiwan’s outlying island, an MND statement said the military will continue to keep a close eye on Taiwan’s sea and land to ensure its security and will take all necessary responsive measures accordingly if needed.