An accident has occurred on one of the U.S. Navy's nuclear powered attack submarines in the Pacific Ocean. The Navy says the sub, for reasons not yet known, hit bottom in one of the deepest parts of the Pacific.

The U.S. Navy says the USS San Francisco was on its way to a port visit in Australia when the accident happened in the Pacific Ocean, about 560 kilometers south of Guam.

Petty Officer Alyssa Batarla, a spokeswoman for the Navy's Pacific Fleet in Honolulu, says the nuclear-powered submarine appeared to have hit the ocean floor in the middle of the East Marianas Basin, causing injuries to some of the 137 crew members.

"There is one critical injury and number of minor injuries," she said. "Approximately 20 personnel [were] injured to the point that they are unable to stand watch. The submarine is currently on the surface and making its best speed back toward its home port in Guam."

The Navy says the submarine's hull is intact and the vessel's nuclear reactor has not been damaged.

Military aircraft, a naval submarine tender and a Coast Guard cutter have been dispatched from Guam to escort the crippled submarine back to its home port. Officials say an attempt will be made to airlift the critically injured sailor from the submarine.

The San Francisco, commissioned in 1981, is a fast-attack submarine which is capable of launching Tomahawk cruise missiles. It is also designed to seek and destroy enemy submarines and surface ships.

The last significant accident involving a U.S. naval attack submarine took place in 2001 when the nuclear-powered USS Greeneville surfaced off Hawaii, colliding with a high school's fishing boat from Japan, killing nine Japanese.