Oak Island officials posted photos on social media after at least six people were stung by the animals, which are closely related to jellyfish.
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Most of the victims were children, Oak Island Water Rescue said.
Officials are urging beachgoers to be on the lookout when entering the water.
Portuguese man o' war have long strands of tentacles and polyps that grow to an average of 30 feet and may extend by as much as 100 feet.
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Their venom is capable of paralyzing and killing small fish and crustaceans.
The man o' war's sting is rarely deadly to people but is painful and causes welts on exposed skin.
Officials also warned that they can sting you weeks after having washed ashore, so steer clear.
They have also been spotted near Holden Beach, officials said.