A US report has confirmed that at least 973 Native American children died while attending federal Indian boarding schools between 1819 and 1969, but acknowledged that "the actual number of children who died while in Indian boarding schools is greater".

Many of the children were buried in at least 74 marked and unmarked burial sites at 65 former schools across the country, according to the report released on Tuesday by the US Department of Interior. The actual number of children who died and the number of potential burial sites are probably greater, the report said.

The report urges the US government to formally apologize for the 150-year-long "deliberate and strategic actions" to assimilate the children and destroy their culture.

The Indian boarding school system was established by the US government to "civilize" and assimilate Native Americans into Anglo-American culture. Children were forcefully taken from their homes and put into boarding schools.

The Federal Indian Boarding School Initiative under the Interior Department has identified by name 18,624 Native children who entered the school system, adding that it is not a comprehensive list.

Additional children may have died after becoming sick at school and being sent home, officials said.

In a letter presenting the report to Interior Secretary Deb Haaland, the first Native American Cabinet secretary, the Assistant Secretary of Indian Affairs Bryan Newland, who led the investigation, wrote: "For the first time in the history of the country, the US government is accounting for its role in operating Indian boarding schools to forcibly assimilate Indian children, and working to set us on a path to heal from the wounds inflicted by those schools."

Newland described one piece of history related to the schools.

"On Dec 28, 1890, the US military entered Third Mesa of Hopi and took 104 children from their families so they could be sent to the Keams Canyon Boarding School. Four years later, on Nov 25, 1894, two US cavalry companies with rapid-fire artillery guns arrived again at Third Mesa to arrest 19 Hopi leaders as prisoners of war after they refused to send additional Hopi children to the school."

Newland said those Indian leaders were imprisoned for almost one year on Alcatraz Island, a former US military installation in San Francisco Bay.

The report also said the US government appropriated more than $23.3 billion in inflation-adjusted dollars between 1871 and 1969 for the federal Indian boarding school system and other similar institutions and associated assimilation policies.

After removal from their Indian tribes and families, many children sent to the boarding schools didn't return home, the report said.

Students' suffering

Children in boarding schools were punished for using their own native language in the schools. Some suffered sexual abuse. The report shared some accounts of the boarding school experiences in the project "The Road to Healing".

"I experience feelings of abandonment because I think of my mother standing on that sidewalk as we were loaded into the green bus to be taken to a boarding school. And I can see it — still have the image of my mom burned in my brain and in my heart where she was crying. What does a mother think? She was helpless," said an Arizona participant.

"Unfortunately, Wrangell was a place that attracted pedophiles and many matrons, men and women, perpetrated themselves upon little boys and girls. … We saw girls going home in the middle of the school year pregnant, and a lot of these children were like 11 and 12, 13 years old," a participant from Alaska stated.

The report cited studies showing that Native children who were separated from their parents suffered both physically and mentally and have had higher rates of suicide and substance abuse compared with the general population.

The report said the US government "should issue a formal acknowledgment of its role in adopting a national policy of forced assimilation of Indian children and carrying out this policy through the removal and confinement of Indian children from their families and Indian Tribes and the Native Hawaiian Community and placement in the federal Indian boarding school system".