Greater efforts are needed in the fight against AIDS to reach at-risk teenagers and children who are failing to get as much help as other communities, US officials warned Monday.

Marking World AIDS Day, US President Barack Obama said in a video address: "We've got to stay focused and committed. Science is on our side, momentum is on our side. It's up to us to finish the job."

The World Health Organization says some 35 million people were living with HIV worldwide in 2013. The virus has killed some 39 million people since it emerged in the 1980s.

But 80 percent of new HIV infections in those countries hardest hit by the virus are among adolescent girls, the State Department said.

And in the United States, "the fastest rate of infection is among 13- to 24-year old gay and bisexual African-American men," said national security advisor Susan Rice.

"Teenagers really. It's a small community with a much higher prevalence of HIV," she said, warning despite progress made in recent years to stamp out AIDS "the finish line is still not in sight."

Some "380,000 adolescent girls are infected each year in sub-Saharan Africa," Rice said, adding the infection rate among young girls was "about four times the rate of boys."

"We're not reaching nearly as many children with anti-viral remedies as we are adults."

So the State Department and the US President's Emergency Plan for Aids Relief (PEPFAR) are teaming up with the Bill and Melinda Gates Foundation and Nike with a $210 million initiative to help cut HIV infection rates among girls and young women.

"Our commitment to fight and end AIDS is unwavering," said Rice.

"We know where we've been, we remember the devastation and the heartbreak, the pain of an HIV diagnosis which was tantamount to a death sentence. And the shameful initial global response.

"And we know where we are going. The promise of an AIDS-free generation and an end to AIDS-related deaths," Rice added.

"An end to the suffering and the stigma of a terrible disease. An end to children born with HIV, their promise tragically cut short from birth. That's a goal that was unthinkable just five years ago."

PEPFAR, set up under previous president George W. Bush, is now supporting anti-retroviral drug treatment for 7.7 million people around the world, many of them in Africa.

It is also providing training for health workers, testing for millions of pregnant women and helping to fund male circumcisions to stop the spread of the virus.

"It is clear that we are at a real turning point," said Secretary of State John Kerry, but he also emphasized that the battle is "not yet won."

"There are major challenges ahead and they will require major commitments if we're going to control the HIV/AIDS pandemic and achieve this AIDS-free generation that is our dream."