"Yet again, we're devastated by a similar tragedy," he said. "It's going to be a very sombre mood at the conference in Melbourne, especially for those of us who have been coming to these conferences for many years." Organisers of the International AIDS Conference, due to begin in Melbourne on Sunday , have not released numbers, but did confirm expected attendees were among the dead. "A number of colleagues and friends en route to attend the 20th International AIDS Conference taking place in Melbourne, Australia, were on board the Malaysia Airlines MH17 flight that has crashed over Ukraine," Michael Kessler of the International AIDS Society (IAS) said in a statement. "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 passenger plane had been flying from Amsterdam to Kuala Lumpur when it was shot down, killing all 298 people on board. It was due to connect with another Malaysia Airlines flight scheduled to arrive in Melbourne on Friday night. Many medical researchers have expressed sympathy online upon hearing that both World Health Organisation (WHO) staff and well-known HIV researchers had died in the crash.

Friends and colleagues of Dutch HIV researcher Dr Joep Lange took to social media to express shock that he was among the victims. Dr Lange had been researching HIV for 30 years. ‘‘He was a kind man and a true humanitarian,’’ US medical professor Seema Yasmin wrote in a series of tweets dedicated to him. ‘‘How do we measure how much a person has done for humanity? People like Joep change the course of epidemics.’’ Dr Yasmin could not be immediately contacted. Also believed to be among the victims was Glenn Thomas, a WHO media adviser. Several of his colleagues have expressed shock over his death on Twitter. UNAIDS executive director Michel Sidibe said he was saddened to hear so many AIDS conference attendees died in the crash.

‘‘My thoughts & prayers to families of those tragically lost on flight #MH17,’’ he tweeted. President-elect of the International AIDS Society, Chris Beyrer, expressed "sincere sadness" at the news that "colleagues and friends" were on board the flight on the way to the 20th International AIDS Conference in Melbourne. "The IAS is hearing unconfirmed reports that some of our friends and colleagues were on board the flight and if that is the case this is a truly sad day." "The IAS has also heard reports that among the passengers was a former IAS president Joep Lange and if that is the case then the HIV/AIDS movement has truly lost a giant." Foreign Minister Julie Bishop said she understood a number of the victims were heading to the AIDS conference, but she had no further details.

Greens Senator Christine Milne expressed her shock at such a loss to the HIV-AIDS community. ‘‘The ramifications go to all the people who are at that conference this weekend and no doubt as more details emerge this tragedy will touch many, many Australians,’’ she told the Senate. Victorian Premier Denis Napthine said MH17 was to connect with MH129 arriving in Melbourne on Friday evening. "Unfortunately, I can now confirm that a number of Victorians are among those who have been killed, although we are not yet in a position to say how many," he said in a statement. "This is a sad and tragic day, not just for Victorians, but for all people and all nations. The shooting down of a passenger aircraft full of innocent civilians is an unspeakable act that will forever leave a dark stain on our history."

He said his office had been in contact with the AIDS conference organisers and the government had offered to co-operate with DFAT to provide assistance to any delegates who require support. Emails sent to several WHO staff prompted out-of-office replies stating they were en route to Melbourne for the AIDS conference. Representatives from WHO could not be immediately contacted. 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. 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.

- with Richard Willingham and Timna Jacks CORRECTION: It has since been determined that six conference delegates were aboard MH17.