Many of the passengers who died when a Malaysia Airlines plane crashed in Ukraine were heading to the 20th International AIDS Conference in Melbourne, officials said Friday.

"My thoughts & prayers to families of those tragically lost on flight #MH17. Many passengers were enroute to #AIDS2014 here in #Melbourne," tweeted UNAIDS executive director Michel Sidibe.

Leading AIDS researcher and former International AIDS Society president Joep Lange is thought to have been on the flight that US officials believe was hit by a surface-to-air missile, killing all 298 people on board.

Australia's National AIDS Trust paid tribute to Lange.

"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.

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.

Held every two years, the International AIDS Conference also is a forum for campaigners to highlight grassroots and financing problems.

It is due to start on Sunday with the 12,000 participants joined this year by former US president Bill Clinton and rock singer and poverty activist Bob Geldof.