Giant jellyfish with tentacles up to 120 feet long are swarming US beaches, with officials warning that even dead creatures can sting.

Swarms of lion's mane jellyfish have turned up along the New England coast, apparently by the thousands.

The creatures have been spotted from Massachusetts’ North Shore to Cape Cod and Nantucket, with some drifting as far north as Maine.

Great Marsh Kayak Tours in Eastham, Massachusetts, said on X: 'Right now, Cape Cod is inundated with lion's mane jellyfish! Multiple thousands of them have gotten swept into the marsh with the tides.'

The creature's tentacles carry a large amount of neurotoxins that cause a sharp, burning sensation. The pain can quickly escalate over an hour and is accompanied by red welts, itching, and potential muscle cramps, headaches or nausea.

The beach town of Beverly in Massachusetts said last week: 'Keep children and pets away from stranded jellyfish. … Do not touch jellyfish or detached tentacles, even if they appear dead.'

The jellyfish is known to sting even 25 days after they have died. 

Experts say the surge in lion's mane jellyfish is being driven by warming ocean temperatures, wind and tidal currents, abundant food sources, and sheltered coastal waters that allow the creatures to thrive before washing ashore.  

Lion's mane jellyfish are among the largest jellyfish on Earth, with some growing tentacles more than 100 feet long. 

Marine experts say this is the largest influx of the species seen in the region since 2020, with unusually high numbers washing ashore since June in communities including Scituate, Marshfield, Hull and Manchester-by-the-Sea. 

A spokesman for the Boston Center for Youth & Families (BCYF) told the Swampscott Tides and WHDH: 'Jellyfish are a natural part of the marine environment, and their presence can vary with tides, currents, and weather conditions.

'While beach operations remain open at this time, we encourage all visitors to remain alert and follow instructions from lifeguards and staff. 

'If you experience a jellyfish sting or require assistance, please notify a lifeguard or staff member immediately.'

The unusually warm start to summer may be helping fuel the jellyfish surge. 

Erica Grow Cei, a spokesman for NOAA's National Weather Service, told the Swampscott Tides that the region averaged 69.2F in June, about six degrees above normal, making it the warmest June in Marblehead since 2001.

'The warmest average June temperature ever observed at Marblehead was 70.3 degrees Fahrenheit in 1999. And, so far, this is the warmest June for Marblehead since 2001,' Cei added. 

 Experts advise rinsing jellyfish stings with seawater, removing any tentacles and seeking medical care if symptoms are severe. 

Although most stings are painful rather than dangerous, some people can experience serious allergic reactions.