As world leaders demand an international investigation into the shooting down of a Malaysian jetliner on Thursday, it has emerged that more than 100 international AIDS researchers, activists and health workers may have been among the passengers of the plane.

The AIDS activists were delegates to the 20th International AIDS Conference, which is due to open on Sunday in Melbourne, Australia, according to a report in USA Today.

Air Malaysia flight MH17 crashed in eastern Ukraine on Thursday afternoon while en route from Amsterdam to Kuala Lumpur, the Malaysian capital. The plane, which had 298 people on board, is believed to have been downed by a missile fired by one of the sides in the ongoing Ukrainian crisis.

There were no survivors from Thursday's crash, which left wreckage and bodies scattered across kilometers of rebel-held territory near Ukraine's border with Russia.

"There's been confirmed a number of senior people who were coming out here who were researchers, who were medical scientists, doctors, people who've been to the forefront of dealing with AIDS across the world," Victoria Premier Denis Napthine told reporters in Melbourne on Friday. "The exact number is not yet known, but there is no doubt it's a substantial number."

The International AIDS Society issued a brief statement Thursday night saying that "a number of colleagues and friends" were on the flight.

The plane’s destination, Kuala Lumpur, is a popular transit point for people travelling to and from Australia and at least 28 Australians are believed to be among the victims.

The tragedy was "a grim day for our country and a grim day for the world," said Australian Prime Minister Tony Abbott. "We bleed for them, we grieve for them and we will do everything we can."

Putin says Ukraine bears responsibility

Meanwhile, pro-Russian separatists and the Ukrainian government are trading charges about who was responsible for the surface-to-air missile that downed the plane. Eastern Ukraine separatist leader Alexander Borodai told Reuters that Ukrainian military forces shot the jet down, but Kiev denied involvement and labeled the incident a "terrorist act."

Pro-Russian separatists in eastern Ukraine are to give international investigators access to the crash site, the BBC reports. The rebels vowed to secure the site and allow the recovery of bodies, according to the Organization for the Security and Co-operation in Europe.

The rebels on Friday said they have found "most" of the plane's recording devices. According to some sources, the so-called black boxes have already been sent to Moscow.

Russian President Vladimir Putin said Ukraine bore responsibility for the crash, but he did not address the question of who might have shot it down and didn't accuse Ukraine of doing so.

"This tragedy would not have happened if there were peace on this land, if the military actions had not been renewed in southeast Ukraine," Putin said, according to a Kremlin statement issued early Friday. "And, certainly, the state over whose territory this occurred bears responsibility for this awful tragedy."

But Ukraine's security services produced what they said were two intercepted telephone conversations that showed rebels were responsible. In the first call, the security services said, rebel commander Igor Bezler tells a Russian military intelligence officer that rebel forces shot down a plane. In the second, two rebel fighters — one at the crash scene — say the rocket attack was carried out by

insurgents about 25 kilometers (15 miles) north of the site.

Neither recording could be independently verified.

Russia's Interfax news agency quoted Donetsk rebel spokesman Sergey Kavtaradze as denying that the intercepted phone conversations were genuine.

But separatist leader Igor Girkin boasted on Facebook at about the same time the plane went down claiming to have downed a transport plane, though the post was quickly deleted after it became clear the plane was a passenger aircraft.

“In Torez An-26 was shot down, its crashes are lying somewhere near the coal mine “Progress,” read the tweet, obtained by FoxNews.com and translated into English. "We have warned everyone: do not fly in our skies.”

The self-titled "Self-defense forces of the Donetsk People's Republic" boasted in a June 29 press release of having taken control of Buk missile defense systems. The Buk, or SA-11 missile launchers, have a range of up to 72,000 feet.

The scale of the disaster could prove a turning point for international pressure to resolve the crisis in Ukraine, which has killed hundreds since protests toppled the Moscow-backed president in Kiev in February and Russia annexed the Crimea a month later. While the West has imposed sanctions on Russia over Ukraine, the

United States has been more aggressive than the European Union in this respect.

Analysts believe the response of Germany and other European powers to the incident could be crucial in deciding the next phase of the stand-off with Moscow.

Reuters journalists saw burning and charred wreckage bearing the red and blue Malaysia Airlines insignia and dozens of bodies in fields near the village of Hrabove.

Makeshift white flags marked where bodies lay in corn fields and among the wreckage.

An emergency worker said at least 100 bodies had been found so far and that debris was spread over 15 km (9 miles).

"I was working in the field on my tractor when I heard the sound of a plane and then a bang," one local man told Reuters at Hrabove, known in Russian as Grabovo. "Then I saw the plane hit the ground and break in two. There was thick black smoke."