LONDON (Reuters) - Rising levels of resistance to HIV drugs could undermine promising progress against the global AIDS epidemic if effective action is not taken early, the World Health Organization (WHO) said on Thursday.

Already in six out of 11 countries surveyed in Africa, Asia and Latin America for a WHO-led report, researchers found that more than 10 percent of HIV patients starting antiretroviral drugs had a strain resistant to the most widely-used medicines.

Once a threshold of 10 percent is reached, the WHO recommends countries urgently review their HIV treatment programs and switch to different drug regimens to limit the spread of resistance.

HIV drug resistance develops when patients do not stick to a prescribed treatment plan - often because they do not have consistent access to proper HIV treatment and care.

Patients with HIV drug resistance start to see their treatment failing, with levels of HIV in their blood rising, and they risk passing on drug-resistant strains to others.

The WHO's warning comes as the latest data from UNAIDS showed encouraging progress against the worldwide HIV/AIDS epidemic, with deaths rates falling and treatment rates rising.

Some 36.7 million people around the world are infected with HIV, but more than half of them - 19.5 million - are getting the antiretroviral therapy medicines they need to suppress the HIV virus and keep their disease in check.

The WHO said, however, that rising HIV drug resistance trends could lead to more infections and deaths.

Mathematical modeling shows an additional 135,000 deaths and 105,000 new infections could follow in the next five years if no action is taken, and HIV treatment costs could increase by an extra $650 million during this time.

"We need to ensure that people who start treatment can stay on effective treatment, to prevent the emergence of HIV drug resistance," said Gottfried Hirnschall, director of the WHO's HIV and hepatitis program.

"When levels of HIV drug resistance become high we recommend that countries shift to an alternative first-line therapy for those... starting treatment."

The WHO said it was issuing new guidance for countries on HIV drug resistance to help them act early against it. These included guidelines on how to improve the quality and consistency of treatment programs and how to transition to new HIV treatments, if and when they are needed.

Reporting by Kate Kelland, editing by Pritha Sarkar

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HIV drug resistance could undermine progress in AIDS battle: WHO - Reuters