The USS Connecticut, Seawolf-class fast attack submarine, struck an unidentified underwater object in the South China Sea on Saturday. The U.S. Navy confirmed the incident on Thursday and said there were several injuries, none of them life-threatening.

“The submarine remains in a safe and stable condition. USS Connecticut’s nuclear propulsion plant and spaces were not affected and remain fully operational. The extent of damage to the remainder of the submarine is being assessed. The U.S. Navy has not requested assistance. The incident will be investigated,” the U.S. Pacific Fleet said in a statement on Thursday.

The Navy did not specify where the incident occurred, but two defense officials told CNN on Thursday it happened in the South China Sea, where a major naval exercise involving the U.S., U.K., Japan, Australia, and other allies is in progress. 

This photo, taken on May 5, 2016, shows crew members of China’s South Sea Fleet taking part in a drill in the Xisha Islands, or the Paracel Islands, in the South China Sea. (STR/AFP via Getty Images)

CNN also noted the increasing tensions between China and Taiwan as a factor in U.S. naval operations.

A Navy official told the Hill on Thursday that injuries from the incident were “bumps and scrapes.” USNI News said, “about 11 sailors were hurt in the incident with moderate to minor injuries.”

A defense official told USNI the Connecticut set course for Guam on Thursday and was expected to arrive on Friday. 

The Seawolf-class attack submarine was designed in the late 1980s as a faster and better-armed successor to the venerable Los Angeles-class sub. Changing naval strategy after the end of the Cold War led the Navy to conclude this class of submarine was too expensive, so only three of them went into service.

USS Connecticut was commissioned in 1998, while the last sub in the class, USS Jimmy Carter, was commissioned in 2005. The nominal crew complement is 116, including 15 officers. They are rated for diving up to 610 meters deep. Their armament typically includes both anti-ship and surface-attack weapons.

Undersea collisions involving modern submarines are rare, but not unheard of. The Japanese Maritime Self-Defense Force (MSDF) submarine Soryu struck a cargo ship while surfacing in February, allegedly due to negligence on the part of the sonar crew.

The last known undersea collision involving a U.S. Navy sub occurred in 2005, when USS San Francisco struck an undersea mountain while moving at flank speed. The collision tossed some crew members 20 feet through the air, killing one and injuring almost all of the others. The San Francisco amazingly survived the impact with relatively minor damage and sailed to Guam for repairs under its own power, returning to service within three years and serving for another eight years afterward.

The attack submarine USS San Francisco is escorted by two harbor tugs as it returns to Apra Harbor, Guam, after a five-month deployment on June 4, 2004. (U.S. Navy photo/Photographer’s Mate 2nd Class Mark A. Leonesio/Released)