When people with HIV don't take their drugs exactly as prescribed, they may have periods where they become infectious. (Ramon Espinosa/Associated Press)

The number of new HIV infections has decreased in recent years, but more than half of the people infected who need life-saving drugs are not getting them, according to a United Nations report.

The World Health Organization and Joint UN Program on HIV/AIDS, or UNAIDS, released their 2009 AIDS epidemic update on Tuesday.

New HIV infections have been reduced by 17 per cent over the past eight years, when the UN Declaration of Commitment on HIV/AIDS was signed.

The decline was credited in part to HIV prevention, but lack of access to support services, medical care and treatment remain a challenge, said Michel Sidibé, executive director of UNAIDS.

"Instead of universal access, what we're experiencing now is universal obstacles," Sidibé told a news conference in Shanghai.

As estimated 33.4 million people are living with HIV worldwide. The figure, which is calculated using a mathematical model, is up from an estimated 33 million in 2007, the UN said.

Last year, HIV infections in sub-Saharan Africa accounted for 72 per cent of all 2.7 million new HIV cases worldwide.

More than 5 million people worldwide need treatment and are not receiving it, Teguest Guerma, acting director of WHO's HIV/AIDS department, told reporters in Geneva.

The report's authors called AIDS a "major public health priority" and called for more funds for antiretroviral drugs, which have saved nearly 3 million lives.

Since the drugs need to be taken indefinitely, the cost of treating HIV will continue to go up.

People with HIV need to take the drugs as prescribed, or there could be times when they become infectious and could spread the virus, said Elizabeth Pisani, an epidemiologist who once worked for UNAIDS.

With files from The Associated Press