Some 8,000 people with HIV in war-torn eastern Ukraine face a critical shortage of medicine and their supply will run out in mid-August unless a blockade is lifted, a UN AIDS envoy has warned.

Speaking ahead of the International AIDS Society conference, which opened in Vancouver, Canada, on Sunday, Michel Kazatchkine called on key nations to intervene as soon as possible.

"I am calling on the United States, Germany, France, Ukraine and Russia to do something," said Kazatchkine, the UN Secretary General's special envoy for AIDS in Eastern Europe and Central Asia.

He said 8,000 patients are "caught in the political crossfire between the Ukrainian government and Russian-supported fighters" because they need both antiretroviral treatments and opioids, which are now blocked at border checkpoints.

The looming crisis is centered in the mostly Russian-speaking Lugansk and Donetsk regions.

The area once had 25 percent of Ukraine's HIV-positive population, but thousands have already fled, said Kazatchkine.

Some 8,000 who are on antiretroviral treatment remain. About 1,000 of them are injection drug users whose treatments for addiction with opioid substitution therapy have been blocked since spring.

He said the treatments are already paid for and the aid group Doctors Without Borders has pledged to deliver and oversee treatment.