London (CNN)A beluga whale has been spotted swimming in the River Thames, sparking fears that the creature is in danger.
The whale was first seen by ecologist Dave Andrews, who tweeted a video of the mammal swimming in the river off Coalhouse Fort, Essex.
“Can’t believe I’m writing this, no joke – BELUGA in the Thames off Coalhouse Fort,” he tweeted.
He added that the whale had been feeding around the barges across the Thames near Gravesend in Kent for around an hour and had not moved more than 200 meters.
Beluga whales, which are easily identifiable by their white markings, usually inhabit the cold Arctic waters off Greenland, Svalbard and the Barents Sea.
According to the UK-based Whale and Dolphin Conservation group, there have been just 20 sightings of beluga whales off the UK coast before today’s incident.
The WDC’s Danny Groves told the UK Press Association, “He or she is obviously very lost and quite possibly in trouble.”
a UK charity that works with whales and dolphins, also urged caution.
“Considering how far the animal is from its range, it may be distressed and so it is vital that onlookers both on land and at sea keep their distance,” she said.
In a tweet, the British Divers Marine Life Rescue, which helps rescue stranded marine wildlife, said it was monitoring the situation.
“Beluga whales are not usually seen in British waters, so it’s very rare,” spokeswoman for the organization Julia Cable said.
According to National Geographic magazine, beluga whales can range from 13 to 20 feet in length. They have a rounded forehead and no dorsal fin.
CNN’s Samantha Tapfumaneyi contributed to this report.