CHATHAM, Mass. -- The state has confirmed that at least one of five sharks spotted off the coast of Cape Cod was a great white shark, and is planning local police boat patrols this weekend.

Shark expert Greg Skomal spotted the great white within a mile of Chatham on Friday, but at least a mile from the closest swimming beach.

Lisa Capone, a spokeswoman for the state Executive Office of Energy and Environmental Affairs, said sharks are common in the area because of the abundance of seals, which sharks eat.

Capone said the state is urging the public to beware. She said out of "an abundance of caution," environmental police would this weekend patrol the eastern shore of the Monomoy Wildlife Refuge, just off the Chatham coast.

The town said it plans to keep the beaches in the area open for the holiday weekend, but will have beach and harbormaster patrols closely monitor the area for sharks. It is also asking boaters to immediately report any sightings.

"I am surprised, but I don't go in the water," one beachgoer said.