Authorities are warning tourists against swimming on Chala That Beach in Songkhla province following sightings of Portuguese man-of-war jellyfish.

Many tourists have reportedly been stung by the sea-drifting creature, which is one of the world's most venomous marine species.

The mayor of Songkhla City has told lifeguards to warn tourists and always have a first aid kit ready. Those stung should be immediately taken to hospital, he said.

He advised people to avoid the beach for a while as the jellyfish are expected to remain there until early April.

The Portuguese man-of-war is recognised by its balloon-like float, which may be blue, violet or pink, that rises up to 15 centimetres above the water surface.

In Thailand, they are occasionally seen in the southern provinces of Narathiwat, Pattani, Songkhla, Krabi and Phuket during the monsoon season. The sting can affect the nervous system and heart, causing severe pain and possibly death. People are strongly advised not to touch them.