WASHINGTON (Reuters) - Divers conducted damage assessments on a nuclear-powered submarine and an Aegis cruiser on Monday as the U.S. Navy began administrative and safety investigations to determine why the vessels collided during a routine exercise over the weekend.

U.S. Fleet Forces Command named Rear Admiral Ann Phillips to lead an administrative investigation into the facts and circumstances of the collision between the submarine USS Montpelier and the cruiser USS San Jacinto about 3:30 p.m. on Saturday.

No one was injured in the collision and there was no damage to the submarine's nuclear propulsion system. Both vessels returned to port under their own power on Sunday.

The investigation by Phillips, commander of Expeditionary Strike Group 2, will examine the cause of the collision and determine any fault, neglect or responsibility, the Navy said in a statement.

Phillip also will identify any shortfalls in procedure and recommend corrective action if needed, it said.

Rear Admiral Joseph Tofalo, commander of Submarine Group 10, was named to direct a Safety Investigation Board. The board will identify hazards and possible causes for the collision and make recommendations to prevent similar incidents in the future.

The Montpelier arrived in port on Sunday at Kings Bay, Georgia, a submarine base north of Jacksonville, Florida. The San Jacinto returned to port at Mayport, Florida, home of a major Navy base near Jacksonville.

Navy divers began assessing the damage to the two vessels on Monday but did not reach a final conclusion, a Navy official said. The assessment was expected to continue for a few more days, the official said.

The Montpelier and San Jacinto were conducting routine training at the time of the accident.

A Navy official said the watch team aboard the San Jacinto saw a periscope rise from the water about 100 to 200 yards (metres) ahead of the vessel during the exercise. The cruiser ordered "all back," but the San Jacinto still collided with the submarine.

The collision caused the collapse of the cruiser's sonar dome, a bulbous-shaped device on the bow of the ship beneath the water line, the Navy official said on condition of anonymity.

The rubber dome houses some of the vessel's electronic navigation, detection and ranging equipment.

The ships were part of a strike group led by the aircraft carrier USS Harry S. Truman. The Truman was in the area and provided support to the vessels.

(Reporting By David Alexander; Editing by Sandra Maler)