Three rabbis and three Jewish groups asked a federal court Thursday to block enforcement of a new regulation requiring written parental consent for a rite called "metzitzah b'peh," in Hebrew, which city health experts said can spread infection and has killed two children since 2004.

During the ritual, the person performing the circumcision attempts to cleanse the wound by sucking blood from the cut and spitting it aside.

The saliva contact puts the infant at increased risk of getting herpes simplex, a virus that is carried harmlessly by a large majority of adults but that can be deadly in newborns.

New York City's Health Department said it has documented 11 cases of the infection since 2000 among children believed to have undergone the ritual. Ten required hospitalization. Two developed brain damage. Two died.

The deaths prompted some doctors to call for the practice to be banned entirely, but the city's Board of Health adopted a compromise approach instead last month. Under the new rule, mohelim performing the circumcision would be required inform parents that the city believed the procedure was dangerous and have them sign a consent form.

No one would collect the forms, and the mohelim would be required to keep them for a year before destroying them.

In their lawsuit filed Thursday, rabbis Samuel Blum, Ahron Leiman and Shloime Eichenstein said the city had exaggerated the potential for harm and infringed on their religious freedom.

If the regulation were to take effect, rabbis "will be forced to serve as the Department's mouthpiece for dispensing opinion and 'advice' that directly undermines the required religious ritual that these mohelim regularly perform, in violation of their rights to freedom of speech and freedom of religious exercise," the suit said.

The lawsuit, joined by the Central Rabbinical Congress of the USA and Canada, Agudath Israel of America and the International Bris Association, also argued that the city's safety studies were flawed and that the procedure, when performed properly, was "very safe." It said compelling the rabbis to warn against the procedure was unconstitutional.

In a statement responding to the suit, the city's health commissioner, Dr. Thomas Farley, called the regulation "lawful, appropriate and necessary."

"The city's highest obligation is to protect its children; therefore, it is important that parents know the risks associated with the practice," he said.

Concerns about the safety of metzitzah b'peh go back to at least the mid-19th century, when most Jews abandoned or modified the rite because of concerns about its role in spreading disease. Today, most Reform, Conservative and modern Orthodox mohels use gauze, or a sterile tube, to pull blood from the wound.

The ritual is still practiced widely, however, in New York City's large population of ultra-Orthodox Jews. After the regulation was adopted in September, some rabbis vowed to ignore it, saying the government had no business regulating a religious practice.