A nuclear submarine (search) ran aground Saturday about 350 miles south of Guam, injuring around 20 sailors and sustaining severe damage, the Navy said.

There were no reports of damage to the USS San Francisco's (search) reactor plant, which was operating normally, the Navy said. One of the sailors suffered critical injuries.

The 360-foot submarine was headed back to its home port in Guam, and the incident was under investigation, said Jon Yoshishige, a spokesman for the Pacific Fleet (search) based at Pearl Harbor.

He said there was no information yet on what the submarine struck.

The extent of the damage would not be known until the submarine arrived at Guam Monday, Yoshishige said.

Details on the sailors' injuries were not immediately available, but Yoshishige said an initial assessment put the number injured at around 20. The sub has a crew of 137, officials said.

Navy (search) and Coast Guard aircraft from Guam were sent to monitor the submarine and assist if needed, the Navy said.

Located west of the international date line, Guam is a U.S. territory about 3,700 miles southwest of Hawaii.