NORFOLK, Va. — The Pentagon said late Saturday that it is investigating why a Navy submarine collided with an Aegis cruiser off the East Coast.

The US Fleet Forces Command said that the submarine USS Montpelier and the Aegis cruiser USS San Jacinto collided at about 3:30 p.m. during routine operations. No one was injured, and the extent of any damage to the vessels was not clear Saturday evening, said Lieutenant Commander Brian Badura of the Fleet Forces Command.

‘‘We have had circumstances where Navy vessels have collided at sea in the past, but they’re fairly rare as to how often they do take place,’’ Badura said.

Navy officials said the collision was under investigation, but declined to offer specifics on what happens next or on where the incident took place.

‘‘If we do have an incident that does take place, there are folks that swing into action . . . to help us make a better, more conclusive explanation of exactly what happened,’’ Badura said.

The Fleet Command said in a news release that ‘‘overall damage to both ships is being evaluated,’’ and that the sub’s propulsion plant was ‘‘unaffected by the collision.’’ Both Navy ships are based at Norfolk, Va., and are operating on their own power, it said.