World AIDS Day
Top AIDS researchers and activists are among those feared dead on Malaysia Airlines flight MH17

The AIDS community is in mourning as more than a third of the 298 passengers killed in the Malaysia Airlines MH17 crash were experts and delegates heading to a major annual conference.

Delegates at a pre-conference in Sydney were told that 108 medical researchers, health workers and activists were on the Boeing 777 that went down near the Russia-Ukraine border, including former International AIDS Society president and "true humanitarian" Joep Lange.

Health researcher Clive Aspin, who is attending the pre-conference, said the news has hit everyone particularly hard, coming 16 years after pioneering AIDS researcher Jonathan Mann also died in a plane crash.

"Yet again, we're devastated by a similar tragedy," Aspin said. "It's going to be a very sombre mood at the conference in Melbourne."

Organisers of the International AIDS Conference, due to begin in Melbourne on Sunday, have not released exact numbers but did confirm expected attendees were among the dead.

"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," the group said in a statement.

Also believed to be among the dead is Glenn Thomas, a WHO media adviser. Several of his colleages have expressed shock at his death on Twitter.

"My thoughts & prayers to the families of those tragically lost on flight #MH17", tweeted UNAIDS executive director Michel Sidibe.

Mark Gettleson, a London-based campaigner, tweeted that AIDS activists were also heading from Europe to Melbourne.

''Several on #MH17 flight were @STOPAIDS activists en route to #AIDS2014 conference in Melbourne, fighting to save lives. Tragic,'' he wrote.

While the medical field mourns the lives of those killed, some experts fear that breakthroughs in HIV/AIDS research will now be hampered by the disaster.

"The amount of knowledge that these people who died on the plane were carrying with them and the experiences they had developed will have a devastating impact on HIV research," Associate Professor Brian Owler, federal president of the Australian Medical Association, told TIME magazine.

The International AIDS Conference is now in its 20th year and has attracted major speakers, including former US president Bill Clinton, Sir Bob Geldof and Indonesian Health Minister Nafsiam Mboi.