The US military has eased restrictions on religious tattoos, hairstyles or apparel that troops can display while in uniform, as long as they do not jeopardise "unit cohesion".
The Pentagon said on Wednesday the rules it unveiled marked the first time the agency had laid out a general policy on the issue.
"The new policy states that military departments will accommodate religious requests of service members unless they have an adverse effect on military readiness, mission accomplishment, unit cohesion, and good order and discipline," spokesman Lieutenant Commander Nate Christensen said in a statement.
As long as a unit's mission is not put at risk or its safety jeopardised, the regulations would allow an exception to standard "clean-cut" grooming standards on religious grounds, including facial hair or other hairstyles, tattoos and piercings that reflect a soldier's spiritual faith.
However, certain apparel or other expressions of religious faith would not be permitted if it "interferes with the wear or proper function of special or protective clothing or equipment," such as helmets, flak jackets or flight suits, Christensen said.
The policy mainly affects Sikhs, Muslims, Jews and members of other groups that wear beards or articles of clothing as part of their religion. It also could affect Wiccans and others who may obtain tattoos or piercings for religious reasons.
But advocacy groups expressed concern that the updated policy does little to protect Sikhs and others from the whims of their commanders.
"What is disappointing is that the presumptive bar on Sikh articles of faith remains", said Amardeep Singh, a spokesman for the human rights organisation Sikh Coalition.
Ibrahim Hooper, a spokesman for the Council on American-Islamic Relations, said he welcomed any move to broaden religious accommodation in the US military.
"We've dealt with this issue on a number of occasions, whether it was with beards or with head scarfs or even in support of the Sikh community on the issue of turbans and skullcaps for the Jewish military personnel," he said.