The world is falling behind in its pledge to reduce HIV/AIDS infections and improve treatment, especially in sub-Saharan Africa, according to a U.N. report released Monday.

The report by U.N. Secretary-General Ban Ki-moon said that "critical challenges remain" if the world is to make good on promises made at a U.N.-sponsored meeting on HIV/AIDS in June 2011.

Funding for anti-HIV/AIDS projects has been inadequate, the report said, and a U.N. goal to halt and reverse the spread of HIV/AIDS by 2015 may not be met.

The report to the U.N. General Assembly said that some $15 billion was available to respond to the HIV pandemic in 2010, but it estimated that $22 billion to $24 billion will be needed by 2015 to achieve the U.N.'s HIV/AIDS targets.

"It will be impossible to achieve global targets without sufficient financial resources," Ban said, calling for public, private and charitable funds.

He urged the international community "to now stand up to meet the commitments it has made."

The report said that more than 7 million people in low- and middle-income countries were receiving antiretroviral treatment in 2011, and the goal is to more than double that total to 15 million people in 2015.

There were an estimated 34 million people living with HIV as of December 2010, about half of them women, the report said.

"Sub-Saharan Africa remains most heavily affected, accounting for 68 percent of all people living with HIV and 70 percent of all people newly infected in 2010," it said.

Death rates have fallen, and new infections are down from a peak in 1997, but many populations remain vulnerable, the report said.

"HIV incidence is rising in Eastern Europe and Central Asia, in the Middle East and North Africa, and in certain Asian countries," it said.

Unprotected sex appears to be a major source of the disease in Africa, while injecting drugs appears to be the epidemic driver in Eastern Europe and Central Asia, it said. It added that male-male sex remained a factor worldwide.

The 2015 targets call for the number of new sexually transmitted infections to decline to about 1 million annually, or about half the 2010 rate. The goal for HIV transmission among people who inject drugs also is targeted to fall by 50 percent, to about 120,000 a year. HIV infections among children — including transmission during pregnancy — would largely be eliminated under the 2015 target, down some 390,000.