WASHINGTON – In a direct response to Michelle Obama’s declared war on childhood obesity, an alliance of major food manufacturers on Monday pledged to introduce new, more healthful options, cut portion sizes and trim calories in existing products.

The Healthy Weight Commitment Foundation, a coalition including Campbell Soup, Coca-Cola, General Mills, Kellogg, Kraft Foods and PepsiCo, will slash 1 trillion calories by the end of 2012 and 1.5 trillion calories by the end of 2015. The 16 members make 20-25 percent of the food consumed in the U.S.

“This is precisely the kind of real private-sector commitment that we need. And I hope that more will follow the example that they’ve set,” Obama said at a news conference.

The announcement is one of the first substantial results of the first lady’s “Let’s Move!” campaign, which aims to end childhood obesity within a generation. The industry’s pledge comes two months after Obama urged food corporations “to move farther, faster” and less than a week after the White House announced the findings of its Childhood Obesity Task Force.

The industry has been under pressure from the first lady and from state and local governments considering junk-food taxes and other anti-obesity measures.

Missing from the announcement were any specifics on the new products or cuts that will be made to existing items. But White House officials stressed that the companies will be held accountable. Each year, their progress will be assessed by the Partnership for a Healthier America, a nonpartisan organization for which the first lady serves as honorary chair. If any one of the companies doesn’t meet its target, all of the companies will be held responsible, White House sources said.

The Robert Wood Johnson Foundation, a nonprofit dedicated to improve Americans’ health, also will track the effort’s impact on childhood obesity. A first report is tentatively slated for 2013.

“What the White House is doing is consistent and relentless,” said Marion Nestle, a professor of nutrition at New York University and a frequent critic of the food industry. “The food companies are having their feet held to the fire for making kids fat. That’s awkward. And it is not good for business.”