In the United States, a vocal movement of "intactivists," or people who oppose male circumcision, is engaged in a fierce debate with doctors over the practice of clipping baby boys' foreskins.

Actor Russell Crowe may be the most famous of them. Earlier this year he declared on Twitter: "Circumcision is barbaric and stupid," before swiftly tweeting sorry to anyone who thought he was "mocking the rituals and traditions of others."

Over the weekend, California's governor blocked a bid by opponents of circumcision to have voters decide if local governments could make it a crime for doctors to perform the procedure unless medically necessary.

But the movement has vowed to keep fighting against a medical practice that is done to about 57 percent of American boys -- down from more than 80 percent in the 1980s according to US health authorities -- yet remains rare in most of Europe, Asia and Latin America.

Defense of circumcision typically tends to come from Jews and Muslims who say it is part of their belief set, though opponents say religious circumcision actually makes up less than one percent of all operations.

Increasingly, support for the practice has come from US doctors who warn of the potential risks of not doing it, including more likely cases of sexually transmitted diseases and HIV.

"Based on the medical evidence, banning infant male circumcision would deprive parents of the right to act on behalf of their children's health," wrote Johns Hopkins University epidemiologists Aaron Tobian and Ronald Gray in Tuesday's Journal of the American Medical Association.

"Three randomized trials in Africa demonstrated that adult male circumcision decreases human immunodeficiency virus (HIV) acquisition in men by 51% to 60%," the pair wrote, also citing research that suggests lower rates of other sex diseases.

"If a vaccine were available that reduced HIV risk by 60%, genital herpes risk by 30%, and HR-HPV (high-risk human papillomaviruses) risk by 35%, the medical community would rally behind the immunization and it would be promoted as a game-changing public health intervention."

The pair acknowledged concerns that some of the Africa studies may not be applicable to the United States, where sexually transmitted infections (STIs) "are a persistent problem" despite high circumcision rates.

However they concluded that in all, evidence to support the practice has "increased substantially during the past five years."

They also raised new concerns about what could be a drop in circumcisions among the poor due to the recent uptick from 16 to 18 states that do not allow Medicaid to cover the cost.

That would adversely affect underprivileged and minority communities, "especially for those at the highest risk for these infections," they wrote.

But opponents, like Georgann Chapin, executive director of the privately funded advocacy group Intact America, say the doctors' arguments are unfounded.

"There is no evidence anywhere that countries with lower circumcision rates have public health problems related to the fact that the men are intact," she told AFP.

And further, since the United States bans any genital modification to little girls, the same protection under the law should be afforded to little boys, she said.

"Could you take your daughter in to the doctor and say 'Cut her finger off?' Of course not. Could you take her to the dentist and say 'Pull all of her teeth out, I am afraid she is going to get cavities?'

"If doctors went by the principle that the baby is the patient and the baby cannot give consent... they would say 'I can't do it.'"

A steady voice in what often becomes an emotional debate should come from the American Academy of Pediatrics, which is at work on a new policy statement about male circumcision to come out early next year.

The group's current position paper takes a middle-of-the-road stance.

"Existing scientific evidence demonstrates potential medical benefits of newborn male circumcision; however, these data are not sufficient to recommend routine neonatal circumcision," it says.

Chapin said her group's supporters intend to press the issue at the AAP's upcoming conference later this month in Boston, and will publish an open letter asking doctors to "put down the knife."

"I think people know this is a really weird and messed up thing to do to children -- but there is such a defensiveness about it," said Chapin.

"I believe what we are seeing here is kind of a last-ditch effort to protect a practice that people in their hearts know is wrong."