This post was updated with additional information on the North Korean submarine force.

A North Korean People’s Navy submarine is missing and presumed sunk, a U.S. official told USNI News on Friday.

The unknown class of submarine was operating off the North Korean coast in the last several days when the submarine went missing.

“About week ago it went missing and the speculation is that it sank,” the official told USNI News.

“The North Koreans have not made an attempt to indicate there is something wrong or that they require help or some type of assistance.”

The official was reluctant to give specific details on the presumed loss of the boat due to sensitivities on how the military was tracking the vessel.

A second U.S. official confirmed the basic facts of the missing submarine.

Though U.S. officials would not confirm the details of where the ship went down, USNI News understands it was likely near one of North Korea’s two submarine bases on the Sea of Japan, also called the East Sea, Joe Bermudez, an analyst focusing on North Korea and advisor to the website 38 North from the U.S.- Korea Institute at Johns Hopkins University, told USNI News on Friday.

North Korea has two primary submarine bases on the its eastern coast in addition to three smaller facilities for costal and midget submarines, Bermudez said.

The North Korean military operates a fleet of about 70 submarines ranging in sizes from midget boats with only a few sailors to larger boats that can hold a crew of up to 30 or 40 and attempted a modernization push since the early 2000s.

According to Bermudez, the submarines are of “reasonable quality” and much or their equipment isn’t military grade but civilian material repurposed for military uses.

However, “they have many problems with maintenance – levels of maintenances standards are lower than most comparable navies in East Asia,” he said.

In the last several years, the Kim Jong Un regime has emphasized its submarine force as a threat to South Korea, scrambling a large percentage of its attack boats in August in its largest show of submarine force to date.

While North Korea has its own domestically produced submarines, the bulk of its fleet is made up of decades old Russian designs with limited capability. However, anti-submarine warfare is a major capability gap in the South’s Republic of Korea Navy. In 2010 the ROK Navy corvette Cheonan was sunk by a torpedo presumably from a North Korean midget submarine killing 46 sailors.