The breakfast fast-food war is heating up.

Taco Bell on Thursday rolled out its new breakfast menu, which includes a waffle taco, across more than 5,500 stores in the U.S. The Irvine-based chain made clear it was going after McDonald's, which dominates sales of breakfast at quick-service restaurants.

The menu's launch included a campaign featuring two dozen Ronald McDonalds (everyday men, not the red-haired clown) chowing down on Taco Bell breakfast items.

McDonald's, however, wasn't going to stand by idly. On Friday, the world's biggest hamburger chain announced it would offer a free small cup of coffee to customers for two weeks beginning Monday.

"This event is McDonald's way of encouraging new guests to try McCafe Coffee while giving our breakfast lovers even more reason to enjoy the great taste of our signature blend," said Greg Watson, senior vice president of McDonald's U.S. menu innovation, in a statement.

For their part, McDonald's wouldn't directly say the free coffee promotion was in response to Taco Bell's new breakfast rollout -- and tongue-in-cheek advertising campaign.

"Our plans to share our great coffee have been brewing for some time," a spokeswoman said in an emailed statement.

Nonetheless, competition among fast-food chains has been growing during the morning hours. Starbucks, for instance, is already revamping its food offerings, hoping to get customers to spend more during their morning visits.

Data by research firm Technomic showed that breakfast sales at limited-service restaurants reached $31.7 billion in 2012, expanding 4.8% a year since 2007 and outpacing growth in any other food-service segment.

Of that pool, McDonald's holds 31% of sales, or $10 billion. Burger King has a 3% to 4% share, while Chick-fil-A has as much as a 2.5% share, according to Technomic.