Burger King has extended an olive branch to longtime rival McDonald's -- an offer to collaborate on a bold new hybrid frankenburger called the McWhopper. But McDonald's has said no.

"Good morning, McDonald's," Burger King said in a full-page "open letter" ad in the New York Times. "We come in peace. In fact, we come in honor of peace. We know we've had our petty differences, but how about we call a ceasefire on these so-called 'burger wars'?"

Burger King proposed that the two chains combine "the tastiest bits of your Big Mac and our Whopper, united in one delicious, peace-loving burger" that they' would sell on Peace Day, September 21.

The proceeds would be donated to Peace One Day, a nonprofit that provides educational resources to thousands of schools in 131 countries.

"All we need from you is a few McDonald's crew members to help combine your ingredients with ours," said Burger King in its ad.

McDonald's (MCD) CEO Steve Easterbrook replied on Facebook that it was a "great idea" and "inspiration for a good cause." But he shot down the idea of a McWhopper and offered a much more vague proposal.

"We love the intention but think our two brands could do something bigger to make a difference," he said. "We commit to raise awareness worldwide, perhaps you'll join us in a meaningful global effort?"

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Easterbrook also urged Burger King to "acknowledge that between us there is simply a friendly business competition and certainly not the unequaled circumstances of the real pain and suffering of war."

He signed off with, "We'll be in touch. P.S. A simple phone call will do next time."

Burger King proposed selling the McWhopper at a pop-up location location in Atlanta, which it says is the halfway point between its headquarters in Miami and McDonald's Chicago headquarters.

The Whopper, which Burger King refers to as "America's favorite burger," is a quarter pound (pre-cooked patty weight) of "savory flame-grilled beef" with tomatoes, lettuce, mayonnaise, ketchup, pickles and sliced onions on a sesame seed bun. It is 650 calories with 37 grams of fat and 1.5 grams of trans fat.

McDonald's describes the Big Mac as "a double layer of sear-sizzled 100% pure beef mingled with special sauce" with American cheese, lettuce, minced onions, pickles on a sesame seed bun. It is 540 calories with 28 grams of fat and 1 gram of trans fat.