Nearly three quarters of the 1.2 million Americans with HIV do not have their infection under control, raising the risk of death from AIDS and transmission to others, said a US study on Tuesday.

One in five people with human immunodeficiency virus are unaware that they have the disease, added the report by the Centers for Disease Control and Prevention, released ahead of World AIDS Day on December 1.

Among people who know their HIV status is positive, only about half (51 percent) get ongoing medical treatment, said the CDC's Vital Signs report.

Of all people in the United States who have HIV, whether they know it or not, 36 percent take antiretroviral therapy and 28 percent have a low amount of the virus in their body.

"The HIV crisis in America is far from over," Jonathan Mermin, director of CDC's Division of HIV/AIDS Prevention, told reporters.

"Today's Vital Signs shows that closing the gaps in testing, care and treatment will all be essential to slowing or reversing the US HIV epidemic."

The CDC defines a viral load as "suppressed" if a person has less than 200 copies of blood-borne virus per milliliter of blood.

Treatment with antiretroviral drugs has been shown to suppress levels of the virus in 77 percent of people who follow the regimen, and studies have shown it can cut the risk of transmission to a partner by 96 percent.

But while new cases of HIV have leveled off at about 50,000 in the United States each year, with 16,000 people dying annually of AIDS, more must be done to make sure people get tested and treated, authorities said.

"We are in a time of new hope for stopping HIV, and this week's World AIDS Day is a time to acknowledge that," CDC director Thomas Frieden said, urging a new push by all levels of government.

"The bottom line is we have the tools to stop HIV from spreading in the individual patient and we have the tools to greatly reduce its spread in communities," Frieden said.

"First is to increase the proportion of people who know their status. Knowledge is power," he told reporters.

"Second, to make sure that people with HIV have every opportunity to remain in ongoing care after they're diagnosed."

Black men who have sex with men are a particularly high-risk group, with African-American gay males accounting for 27 percent of all new infections in the United States, according to CDC data.

Also, more than a third of young black homosexual men are infected with HIV, more than twice the level seen in white gay men. Surveys also suggest a full 60 percent of black gay men living with HIV do not know they are HIV positive.

"We are very concerned about the very troubling statistic of black gay men in the US," said Mermin.

Kevin Fenton, director of the National Center for HIV/AIDS, Viral Hepatitis, STD and Tuberculosis Prevention, said high risk groups should get tested at least once year, or more.

"In fact, recent CDC data suggests that gay and bisexual men might even benefit from testing as often as every three to six months," he told reporters.

"There are many reasons why individuals may not get tested for HIV. Some may not think they are at risk, others may want to avoid the stigma of HIV and testing and some may fear learning that they are HIV positive.

According to the latest UN figures, about 34 million people in the world were infected with HIV/AIDS in 2010 and 1.8 million people died. Antiretroviral drugs were credited with saving 700,000 lives last year.