They can eat a duckling and chomp on a small alligator. They are American bullfrogs, and they're over here.

Not only are the frogs three times the size of British toads, they are eating machines that could destroy Sussex wildlife.

People have been warned to look out for the killer frogs, which can grow up to 15in in length and have huge mouths.

Southern Water has put its 200 pond wardens on alert to spot the creatures, which feast on anything from rats to fish.

Diana Smith, regional pond warden co-ordinator, said: "We ask our volunteers to be extra vigilant when they are surveying ponds. It is bad news for our native wild animals if alien creatures such as bullfrogs, terrapins and turtles are released into the wild."

English Nature is particularly concerned with the problem because bullfrogs also spread disease.

The organisation uncovered a large area on the Sussex and Kent border where the frogs had been breeding last year.

The undisclosed region is thought to be under control although experts are worried they could have spread further into Sussex.

English Nature amphibian scientist Jim Foster said: "We have been controlling an area on the border of the two counties after we had reports they had been breeding there.

"We have got rid or caught most of them by draining the ponds in that area. We have had to evacuate 7,000 animals on that site.

"I would not be surprised if they have gone further afield as the frogs can travel very long distances.

"I would expect there to be more reported sightings. The major reason to worry is if there is a male and female together."

The frogs, which originate from the east coast of the United States and Canada, breed at a phenomenal rate - females can lay 20,000 eggs a year.

Mr Foster said: "They are effective predators and can munch through fishes, invertebrates and small mammals. It has been known for them to eat baby alligators.

"They were introduced to this country in two ways. Firstly, they have been illegally imported since the Eighties and then escaped into the wild. They may have also got into the country through the importation of fish.

"If people see them, they should contact English Nature."

English Nature can be contacted on 01733 455000.