World AIDS community mourned several passengers on a crashed Malaysian Airlines plane who were heading to Australia for a global conference on the epidemic.
Flight MH17 from Amsterdam to Kuala Lumpur, which US officials believe was hit by a surface-to-air missile, killing all 298 people on board, was due to connect with another flight to Melbourne.
"My thoughts & prayers to families of those tragically lost on flight . Many passengers were enroute to here in ," tweeted UNAIDS executive director Michel Sidibe.
Held every two years, the International AIDS Conference to start on Sunday is a forum for campaigners to highlight developments in fighting the disease and discuss financing problems.
The conference this year was also expected to channel anger about laws in Africa that stigmatise homosexuality and in the former Soviet Union that punish intravenous drug users -- a crackdown now extended to Russian-annexed Crimea.
Some 12,000 participants are due to take part, joined by former US president Bill Clinton and rock singer and poverty activist Bob Geldof.
Leading AIDS researcher and former International AIDS Society president Joep Lange is thought to have been on the flight that went down in strife-torn eastern Ukraine.
It is unclear who fired the weapon although Australian Prime Minister Tony Abbott said it appeared to have been Russian separatists.
- 'Desperately sad news' -
Australia's National AIDS Trust paid tribute to Dutchman Lange, who has been involved in HIV research and treatment since 1983.
"Reports Joep Lange died in Malaysian plane crash today, with other scientists on way to @AIDS_conference. Desperately sad news," it said on Twitter.
American academic and AIDS activist Gregg Gonsalves tweeted that "lots of AIDS researchers, activists, officials on downed Malaysia Airlines flight to Melbourne for Intl AIDS Conference", naming Lange.
"Joep Lange was a leading AIDS researcher and clinician and an activist at heart. Lost today too soon on Malaysian flight 019 (sic). RIP," he said.
US doctor Seema Yasmin described him on social media as "a kind man and a true humanitarian".
"How do we measure how much a person has done for humanity? People like Joep change the course of humanity," she said.
The International AIDS Society confirmed in a statement that "a number of our colleagues and friends" en route to the conference were on Flight MH17.
"At this incredibly sad and sensitive time the IAS stands with our international family and sends condolences to the loved ones of those who have been lost to this tragedy," it said.
Of the 298 onboard the plane, 154 were Dutch nationals. There also were 43 Malaysians and 27 Australians, as well as passengers from Indonesia, Germany, Belgium, the Philippines, Britain and Canada.
London-based AIDS campaigner Mark Gettleson said some of those on the flight were from the STOPAIDSNOW group, which works to offer care and treatment to those affected by HIV and AIDS, and support prevention initiatives.
"Several on flight were activists en route to conference in Melbourne, fighting to save lives. Tragic," he tweeted.
The flight took off from Schiphol airport in Amsterdam shortly after noon Thursday and was supposed to land in Kuala Lumpur early Friday. A scheduled connection to Melbourne was due to arrive Friday evening.
Some 35 million people live with HIV although global AIDS-related deaths and new infections have fallen by over a third in a decade, raising hopes of beating the killer disease by 2030, the United Nations said Wednesday.