HONOLULU -- Waikiki lifeguards on Saturday are closing a beach because of an infestation of box jellyfish.

They also are warning users of other Oahu beaches to watch out for the critters, which can deliver a nasty sting.

Some 2,300 jellyfish were estimated at Waikiki beaches on Saturday and another 1,000 at Ala Moana shores. More than 20 stings have been reported, and ambulances responded to two incidents.

The monthly influx of the critters began Friday when about 400 were spotted.

Hanauma Bay Nature Preserve was closed on Saturday. Warning signs were posted at Ala Moana Beach Park, Diamond Head Beach Park, Duke Paoa Kahanamoku Beach Park, Kapiolani Beach Park, Kuhio Beach Park and San Souci Beach.