COVID-19 could cause an additional half a million AIDS deaths if treatment is disrupted long term, the United Nations said Monday in a warning that the pandemic was jeopardizing years of progress against HIV.

At the start of a week of virtual International AIDS Conferences, the UN said the world was already way off course in its plan to end the public health threat even before COVID-19.

Although AIDS-related deaths have fallen by 60 percent since the peak of the HIV epidemic in 2004, around 690,000 still died from the illness in 2019.

Around 1.7 million people were infected last year, and there are now close to 40 million people living with HIV worldwide.

The UN's annual report said that the 2020 target of reducing AIDS-related deaths to fewer than 500,000, and new HIV infections to under 500,000 will now be missed.

Millions of people died in recent decades despite the existence of effective treatments, it said, calling on the world to learn lessons from the AIDS epidemic in its COVID-19 response.

"Like the HIV epidemic before it, the COVID-19 pandemic is exposing our world's fragilities – including persistent economic and social inequalities and woefully inadequate investments in public health," said UN Secretary General Antonio Guterres.

Key populations at high-risk of HIV/AIDS are being put in further danger as lockdowns and distribution of medicines leaves them "even more vulnerable than usual," the report said.

Research released Monday showed the pandemic was already impacting access to preventative medicine (PrEP) among communities at risk.

At one Boston medical center, a survey of more than 3,500 patients on the PrEP program showed that lapses in picking up repeat medication had risen 278 percent in the first four months of 2020.

Year on year, the overall number of patients receiving PrEP had fallen 18 percent, the research showed.

WHO Director General Tedros Adhanom Ghebreyesus said the findings were "deeply concerning."

"We cannot let the COVID-19 pandemic undo the hard-won gains in the global response to this disease," he said.

"As we have seen with HIV, and as we now see with COVID-19, epidemics affect everyone but they do not affect everyone equally. To fight COVID-19, we have to fight inequality. Because pandemics feed on and widen inequalities," said UNAIDS Executive Director Winnie Byanyima.

The report pointed out that out of the therapies to treat AIDS, anti-retroviral therapy has particularly shown its unequal access. Anti-retroviral therapy, also called ARVs, is a life-saving treatment for HIV patients.

Increased access to ARVs has averted an estimated 12.1 million AIDS-related deaths since 2010. However, 690,000 died of AIDS-related illnesses last year and 12.6 million out of the 38 million people living with HIV were not accessing the life-saving treatment.

"Stigma and discrimination and widespread inequalities are major barriers to ending AIDS. Countries need to listen to the evidence and step up to their human rights responsibilities," said Byanyima.

(CGTN's Liu Yang also contributed to this story.)