A leading US health official has warned that a campaign started by a children's hospital to fight childhood obesity poses health risks.
Alan Guttmacher, a child health expert at the National Institutes of Health, says Atlanta-based Strong4Life "carries a great risk of increasing stigma".
The campaign has been criticised for using stark images of overweight children to warn of obesity risks.
The hospital behind the campaign said it raised awareness of obesity dangers.
The US state of Georgia has the nation's second highest childhood obesity rate.
Dr Guttmacher's response comes after online activists mounted a month-long counter-campaign.
In a letter to fat activist Shannon Russell, Dr Guttmacher, director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, said childhood stigma "can reinforce unhealthy behaviours".
Stigma also "poses risks to the psychological health" of obese adolescents, Dr Guttmacher wrote.
The Strong4Life campaign, run by Children's Healthcare of Atlanta, has posted billboards in the Atlanta area since August featuring unflattering black and white photos of unhappy fat children.
The posters are accompanied by a warning banner and messages such as: "Chubby isn't cute if it leads to type two diabetes"; and "Being fat takes the fun out of being a kid".
'Complexity of factors'
Those behind the campaign defended the advertising, insisting there is a public health issue at stake.
"We saw the problem as something that we should take some responsibility for, and something that we had to fix," Mark Wulkan, surgeon-in-chief at Children's Healthcare of Atlanta, told the BBC.
"When we looked at how do you get that awareness, really the most effective means are to use techniques that some might say are controversial," says Dr Wulkan.
The Strong4Life ads were partly modelled on a successful anti-methamphetamine campaign in the state.
But online activists saw the billboards as bullying, and said they play up dangerous stereotypes about health and size.
As part of a campaign against the ads, Shannon Russell, who runs the blog Fierce, Freethinking Fatties, solicited letters denouncing the campaign from various health care organisations, including the National Institutes of Health.
After first being told that the NIH did not offer comment on private health campaigns, Mr Russell says he received an emailed letter on 8 February from Dr Guttmacher.
The letter, which was provided to the BBC, expressed concern about the risk of the campaign and stigma to overweight and obese children, and cited the complex factors that led to obesity.
"[S]tudies show that the perception that obesity is solely a matter of personal responsibility as opposed to understanding the complexity of contributing factors, can increase negative stereotypes about overweight people.
"It is important, therefore, that public messages about obesity address this complexity whenever possible."
The letter also detailed the many ways in which the NIH was working to address the causes and consequences of obesity.
"Addressing obesity in the US is a clear priority for our department," it read. "We strongly support programs and public health messages that are based on carefully conducted research."
Dr Wulkan said the hospital did significant market testing before the adverts were posted, and defended the campaign as a necessary tool in fighting for healthier children.
"Our intent was never to stigmatise the children," he said.
A spokesperson for Children's Healthcare of Atlanta said that most of the billboards had come down as the campaign moved into its next phase, but some still remained in neighbourhoods with high childhood obesity rates.