WASHINGTON (AP) — A U.S. Navy attack submarine struck an object while submerged in international waters in the South China Sea, officials said Thursday. They said there were no life-threatening injuries and the sub was still fully operational.

In a brief statement providing few details of an incident that happened five days ago, U.S. Pacific Fleet said the USS Connecticut remained in a “safe and stable condition.” The Seawolf-class submarine's nuclear propulsion plant was not affected, it added.

“The extent of damage to the remainder of the submarine is being assessed,” the statement said, adding that the incident will be investigated.

The statement did not specify the location of the incident, but two Navy officials, speaking on condition of anonymity to discuss details not announced publicly, said it happened in the South China Sea while the Connecticut was conducting routine operations. The officials said the sub then headed toward port at Guam. They said the incident was not announced before Thursday in order to maintain operational security.

The officials said it was not yet clear what object the sub had struck but that it was not another submarine. One official said it could have been a sunken vessel, a sunken container or other uncharted object.

Two sailors aboard the Connecticut sustained moderate injuries and about nine others had minor injuries like scrapes and bruises, the officials said. All were treated aboard the sub.

The attack boat is now headed to Guam and was expected to pull in within the next day, a defense official told U.S. Naval Institute News. The submarine has been making its way to Guam on the surface since Saturday, the USNI News reported.

The last known instance where a submerged U.S. submarine struck another underwater object — the USS San Francisco (SSN-711) struck an underwater mountain at full speed near Guam — was in 2005, USNI News reported. One sailor died in that incident.

Electric Boat won the contract to build the boat, the second Seawolf-class submarine, in 1991. Later named the Connecticut, the boat was christened at EB in 1997 and delivered to the Navy in 1998.

The Navy awarded EB a $5.2 million contract for modernization work on the boat in 2005.

The collision occurred during a busy time in the Pacific region, with the USS Carl Vinson, an aircraft carrier, and its escort ships moving through the South China Sea in recent days. The Carl Vinson more recently was spotted in the Philippine Sea, operating with two other aircraft carriers, the USS Ronald Reagan and the British HMS Queen Elizabeth.

The USS Connecticut is 353 feet long, and typically carries a crew of about 15 officers and 100 enlisted sailors, according to Navy fact sheets. It's part of the service's Seawolf class of submarines, a Cold War-era fleet designed to chase Soviet subs. The class was expected to include up to 30 vessels, but only three were built after the Cold War ended and the Pentagon's priorities shifted, according to a Congressional Research Service report.