A global AIDS summit was in shock Saturday at the loss of colleagues in the Malaysia Airlines disaster over Ukraine, but spirits were lifted when the number who died was put at six, far fewer than feared.

Reports on Friday said as many as 100 passengers on the plane were en route to the 20th International AIDS Conference in Melbourne when it went down in a rebel-held part of the country on Thursday, killing all 298 on board.

But International AIDS Society president Francoise Barre-Sinoussi said just six attendees were confirmed dead.

"The number that we have confirmed through our contacts with authorities in Australia, in Malaysia, and Dutch authorities as well is six people. It may be a little bit more, but not the numbers that have been announced," she said.

Those killed include prominent Dutchman Joep Lange, a pioneer of cheap anti-retrovirals for the poor who had been involved in HIV research and treatment since 1983.

Officials said Pim de Kuijer from STOPAIDSNOW was also on board, along with Lucie van Mens, director of AIDS Action Europe and her colleague Maria Adriana de Schutter.

World Health Organization official Glenn Thomas and Jacqueline van Tongeren from the Amsterdam Institute for Global Health and Development also died in the crash.

Flight MH17 from Amsterdam to Kuala Lumpur, which US officials believe was hit by a surface-to-air missile, was due to connect with another flight to Melbourne.

Despite the death toll not being as bad as feared, the sense of loss was palpable as attendees gathered at the Melbourne Convention Centre on the eve of the high-powered meeting, the largest of its kind in the world.

"The extent of our loss is hard to comprehend and express," Barre-Sinoussi, a Nobel prize winner, said ahead of the summit that opens on Sunday.

"Our colleagues were travelling because of their dedication to bringing an end to AIDS.

"We will honour their commitment and keep them in our hearts as we begin our programme on Sunday. This tragedy is probably a good sign to work again together and to continue as a tribute for our colleagues."

The loss of Lange was felt particularly hard by Barre-Sinoussi.

"Joep was not only a great researcher, a great champion of the fight against HIV for many years, he was also a wonderful human being," she said.

"He was firmly believing that a cure for HIV was possible, as we all do, and was one of the first supporters of the idea of integrating social science with the search for a cure."

Officials from the PharmAccess Foundation, which Lange launched in 2000 to facilitate access to treatment for HIV and AIDS patients across Africa, said his death was "a massive loss".

- Clinton: Victims are 'martyrs' -

Some 12,000 participants are due to take part in the conference, including former US president Bill Clinton, who told CNN that those who died were "martyrs".

"Thinking about those people, knocked out of the sky, it's pretty tough," he said, adding that "they were doing so much good".

"Those people are really, in a way, martyrs to the cause that we are going to Australia to talk about."

Clinton is due to give an address in Melbourne on Wednesday.

Held every two years, the International AIDS Conference is a forum for campaigners to highlight developments in fighting the disease and discuss financing problems.

It is this year 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 35 million people live with HIV, although global AIDS-related deaths and new infections have fallen by more than a third in a decade, raising hopes of beating the killer disease by 2030.