A US nuclear-powered submarine struck an object underwater in the South China Sea on Saturday, according to two defense officials. A number of sailors on board the USS Connecticut were injured in the accident, the officials said.

According to CNN, None of the injuries were life-threatening, according to a statement from US Pacific Fleet. It’s unclear what the Seawolf-class submarine may have hit while it was submerged. “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 statement said.

“The incident will be investigated.” The US Navy did not specify the incident took place in the South China Sea, only that it occurred in international waters in the Indo-Pacific region. The accident happened as tensions between the US and China soared over the Chinese military’s incursions into Taiwan’s Air Defense Identification Zone (ADIZ).

According to TheGuardian, In a brief statement on Thursday that provided few details of the incident, which happened five days ago, US Pacific Fleet said the USS Connecticut remained in a “safe and stable condition”, that there were no life-threatening injuries and the sub was still fully operational.

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 another uncharted object.