The Indian Health Service announced this week that Black Native Americans in the Seminole Nation, known as the Freedmen, will now be eligible for health care through the federal agency, which previously denied them coronavirus vaccinations and other care.

The shift in policy comes as the Biden administration and members of Congress are pressuring the Seminole and other Native tribes in Oklahoma to desegregate their constitutions and include the Freedmen, many of whom are descendants of Black people who had been held as slaves by the tribes, as full and equal citizens of their tribes under post-Civil War treaty obligations.

“The I.H.S.-operated Wewoka Indian Health Clinic provides services to members of the Seminole Nation of Oklahoma, and personnel at the clinic and other I.H.S. facilities in Oklahoma have been informed that they should provide services to Seminole Freedmen who present at their clinics and hospitals,” the Indian Health Service said in a statement.

The Seminole Nation did not immediately respond to a request for comment on the announcement.

The I.H.S. system consists of 26 hospitals, 56 health centers and 32 health stations throughout the country that provide health care to 2.6 million Native Americans. The I.H.S. clinic in Wewoka provides care for the Seminole Nation, whose headquarters are there.

The Choctaw, Muscogee (Creek), Cherokee, Seminole and Chickasaw Nations, which originally inhabited the Southeast, purchased enslaved Black people as laborers in the 18th and 19th centuries, and took them along when the federal government forcibly moved the tribes to Indian Country, now the state of Oklahoma. Thousands of Freedmen currently live there.

The Seminole Nation currently grants only limited citizenship to Freedmen, many of whom are poor and live in isolated areas where I.H.S. clinics may be the only health care option. They can vote and hold some elected offices under the tribe’s constitution, but have not been eligible for a number of tribal services — including housing, health care and education — many of them funded by the federal government.

“It brings tears to my eyes,” Reggie Knighton, the chief of the Dosar-Barkus Band — one of the Freedmen tribal bands in the Seminole Nation — said of the announcement by the I.H.S. “I just won’t feel any better until we get all the rights that we’re entitled to.”

Mr. Knighton and other senior members of the Freedmen bands, including two representatives in the tribe’s legislature, were denied Covid vaccinations by I.H.S. clinics earlier in the year. Mr. Knighton ended up getting vaccinated at a nearby Walmart pharmacy, he said.

Dora Thomas, 82, a former council representative for the Seminole Nation, tried to get a vaccination from the I.H.S. after being hospitalized with Covid-19 in January along with her husband, who died.

Ms. Thomas’s son, Patrick Thomas, said he called the I.H.S. clinic in Wewoka the next month to schedule vaccinations for him and his mother. They were denied, he said, because they were Freedmen.

“When it got to that point, I felt like, ‘Man, you all hate us that bad, I don’t even trust you to give me a shot now,’” said Mr. Thomas, who is also a former council representative.

The denial of health services to Freedmen came up during a hearing on their status in July, provoking a furious reaction from Representative Maxine Waters, Democrat of California and chairwoman of the House Financial Services Committee.

“People died, including leaders of the Freedmen people,” Marilyn Vann, a Cherokee citizen and president of the Descendants of Freedmen of the Five Civilized Tribes Association, said.

“I don’t know what else to say,” Ms. Waters replied after a stunned silence.

In a statement, I.H.S. said the Wewoka Indian Health Center made the vaccine available to Freedmen on March 1 — two months after the center began offering it to tribal members.

It is unclear how many Freedmen were denied vaccinations by the I.H.S. The coronavirus has torn through the ranks of tribal elders in Oklahoma, and the pandemic has killed American Indians and Alaska Natives at nearly twice the rate of white Americans.

The I.H.S. and the Seminole Nation have blamed each other for the denial of services. The health agency said in June that the agency “has no role” in determining whether the Freedmen were eligible for its services. In March, the Seminole Nation’s chief said the tribe does not operate the I.H.S. clinics and has “no policy oversight” on the Freedmen’s eligibility.

Post-Civil War treaties in 1866 gave the formerly enslaved Freedmen of the Seminole and other tribes in Oklahoma full rights of tribal citizenship. But in practice, the Freedmen have often been segregated within the tribes, and their political rights have eroded over time.

The Seminole Nation voted in 2000 to strip their Freedmen of tribal citizenship, but the nation reversed itself after the federal Bureau of Indian Affairs withheld funding from the tribe in response. Now, Seminole Freedmen are classified as having no “Indian blood,” segregating them from blood citizens of the tribe who can be elected to senior leadership positions and are eligible for financial assistance.

Documents obtained by The New York Times show that The Seminole Nation of Oklahoma intentionally excluded the Freedmen from receiving a one-time payment of $2,000 through a federal coronavirus relief program, the American Rescue Plan, by requiring all applicants to be an “enrolled member by blood of the Seminole Nation of Oklahoma.”

The same blood requirement was also used to deny Freedmen access to last year’s federal Covid-19 Emergency Assistance Program because they didn’t hold “valid” tribal cards, according to the documents. Seminole Freedmen are given tribal membership cards stating they have “voting benefits only.”

A bill introduced in the Seminole Nation legislature that would make the tribe’s Freedmen eligible for the American Rescue Plan funds was voted down, 12 to 15, last month.

Other tribes in Oklahoma — like the Choctaw and Muscogee (Creek) Nations — have expelled their Freedmen entirely by changes in their tribal constitutions that added “by blood” requirements for citizenship. The Chickasaw Nation jointly signed its Reconstruction treaty with the Choctaw Nation, but never enrolled its Freedmen as citizens.

Freedmen and “by blood” members of Native tribes in Oklahoma were listed separately by the federal government in the Dawes Rolls of 1906; descendants of people on both lists are supposed to be eligible for tribal membership, according to treaties the tribes signed with the federal government.

The tribes changed their constitutions over time to expel Black tribal members descended from the Freedmen rolls. Only one tribe in Oklahoma, the Cherokee Nation, has fully reversed those policies and desegregated its constitution in February this year.

Now two branches of government are putting pressure on the other tribes to comply with their treaty obligations. In May, Deb Haaland, the first Native American secretary of the Interior, called for tribes in Oklahoma to follow the Cherokee Nation’s example and voluntarily amend their constitutions to remove the racial qualifications that had segregated and expelled the Freedmen.

A legislative provision that could be included in the House version of President Biden’s $3.5 trillion social policy bill would also penalize tribes that continue to exclude the Freedmen — giving the Interior Department the authority to withhold tens of millions of dollars in federal funding from tribes that do not comply.

Last year, a nurse at the Indian Health Service clinic in Wewoka glanced at LeEtta Osborne-Sampson’s tribal identification card and denied her a shot, Ms. Osborne-Sampson said, because it said she was a Freedmen. Ms. Osborne-Sampson, who also sits on Seminole Nation’s tribal council, demanded to know why she was being denied services. The nurse called the tribal police, who asked her to leave.

The I.H.S. said it was “not aware of such an incident occurring.”

Ms. Osborne-Sampson said experiences like hers were all too common for Seminole Freedmen seeking health care during the pandemic, and that they contributed to deaths in the Freedmen community.

Despite the victory, Ms. Osborne-Sampson said a battle remains in order for the Seminole Freedmen to be treated as equal tribal citizens.

“We should be treated as equals in this nation,” Ms. Osborne-Sampson said. “We already suffer in this country and aren’t treated as equals. Why should we keep being treated as second-class citizens in our tribe?”