WASHINGTON (AFP) - A "monster" combo of a peppercorn burger, bottomless steak fries and a salted caramel milkshake has been named the single unhealthiest meal in the United States.

The Center for Science in the Public Interest, announcing its Xtreme Eating Awards for 2014, said restaurant chain Red Robin's meal contains a whopping 3,540 calories altogether.

"To dispose of those calories, the average person would need to walk briskly for a full 12 hours," the advocacy group said.

The centre also declared the innocently named "bruleed French toast" at another national restaurant chain, Cheesecake Factory, America's most caloric.

The dish - custard-soaked bread topped with powdered sugar, served with bacon and maple syrup - packs 2,780 calories.

It faces some stiff competition in specialty burger restaurant PYT's limited edition Donut Cheesesteak Burger: a five-ounce, all-beef patty dripping with cheddar cheese and fried onions - served between a glazed donut.

The Philadelphia restaurant says its latest concoction is a novelty take on the city's signature classic, the Philly cheesesteak sandwich.

"It definitely has sweet and savoury elements to it," executive chef Kim Malcolm told AFP by telephone.

"The donut is sweet, but not overpoweringly so, (and) the real beefy flavor cuts through."

On the menu as PYT's "burger of the week", the US$15 (S$18.70) creation is finding a following, with 13 sold by 12.15pm Wednesday local time - 40 minutes after opening - amid social media buzz.

"If everybody likes it, we'll keep it around. If nobody likes it, we'll find the next big thing," Malcolm said, adding that her team has yet to tally up the burger's calorie count.