Burger King says it will start offering a "French Fry Burger" for a buck, as it looks to fend off McDonald's aggressive push for its Dollar Menu.

The Miami-based chain says the burger will be available Sept. 1 through the fall, as it looks to drum up sales and customer interest with cheap new concoctions.

The burger, which clocks in at 360 calories and 19 grams of fat, is a relatively novel offering, but doesn't require any extra investment from Burger King; it's basically a standard beef patty topped with four of the chain's french fries.

The offering comes as fast-food chains jump through hoops to get customers through their doors, in large part by heavily promoting their cheapest eats.

McDonald's in particular has stepped up advertising for its Dollar Menu, which now features its Grilled Onion Cheddar Burger and a more substantial McDouble, which has two patties and a slice of cheese. That forced Burger King and Wendy's to adjust their tactics earlier this year to stress value more clearly as well. Burger King, for example, has offered any two sandwiches for $5 and a Junior Whopper for $1.29.

It's not clear whether the strategy will pay off for the chains, or what impact it could have on wearing away at profit margins over the long term. But in the latest quarter, Burger King said its sales at North American restaurants open at least a year slipped 0.5 percent. Wendy's reported an anemic 0.4 percent rise, while McDonald's reported a 1 percent increase for U.S. restaurants. The metric is an important indicator of health because it strips out the impact of newly opened and closed locations.

Other chains, by contrast, are faring better. Chipotle reported a 3.4 percent increase and Starbucks said the figure rose 9 percent.