Global efforts to prevent the spread of HIV infection must be stepped up after a long-term decline in the number of new cases in adults stalled, the United Nations AIDS agency warned on Tuesday.

In a report on the worldwide epidemic, UNAIDS said an estimated 1.9 million adults had become infected with HIV every year for at least the past five years. Globally, some 36.7 million were now infected.

New HIV infections among adults were now rising in Eastern Europe, central Asia, the Caribbean and Middle East and North Africa, the report said.

"We are sounding the alarm," said Michel Sidib��, executive director of UNAIDS. "The power of prevention is not being realized. If there is a resurgence in new HIV infections now, the epidemic will become impossible to control. The world needs to take urgent and immediate action."

The epidemic of the Human Immunodeficiency Virus (HIV) that causes AIDS has had a devastating impact since it began some 35 years ago. Since then, 35 million people have died from AIDS-related illnesses and an estimated 78 million have become infected with HIV.

The World Health Organization says all people diagnosed with HIV should have immediate access to antiretroviral AIDS drugs, which hold the virus in check and give patients a good chance of a long and relatively healthy life.

But the UNAIDS report found only around 17 million patients worldwide - less than half of those diagnosed as HIV positive - are currently on treatment.

The UNAIDS report said significant progress had been made in stopping new HIV infections among children, with new cases down by more than 70 percent since 2001.

But the decline in new infections among adults had stalled, at a time when donor funding for the fight against AIDS had dropped to its lowest levels since 2010, it said.

International donor contributions fell from a peak of $9.7 billion in 2013 to $8.1 billion in 2015.

(Reporting by Kate Kelland; editing by John Stonestreet)