CANBERRA, July 18 -- Australian Prime Minister Tony Abbott has described as a "terrible tragedy" the shooting down of Malaysia Airlines flight MH17 that has claimed the lives of as many as 27 Australians.

Speaking on radio on Friday morning, Abbott said it seemed certain that the Malaysia Airlines plane had been "shot down," saying if this was the case it would be an "unspeakable crime."

"If it does turn out that this aircraft was brought down by a surface-to-air missile, there is no doubt this would be,under those circumstances, an unspeakable crime and the perpetrators should swiftly be brought to justice," he said.

However, he emphasized that there were no hard facts at the moment.

The plane was en route to Kuala Lumpur from Amsterdam when it is believed to have been brought down over separatist-controlled eastern Ukraine.

Dutch authorities have confirmed 27 Australians were among the 298 passengers on board the plane, but Abbott said the Department of Foreign Affairs and Trade in Canberra had still only confirmed the presence of 23 Australians on the plane.

Some of the passengers were due to fly onwards to Melbourne, many of them to attend next week's international AIDS conference in the city.

The prime minister offered his deepest condolences to the families and friends of the people involved in what he said was a "terrible tragedy."

"We can assure families that, at the highest levels of government, we feel for them, we grieve with them and we pray for them," he said on Friday.

"That's the first thing we do. The second thing is to try to provide whatever practical assistance we can, in terms of counsel and the repatriations of remains and so on."

The National Security Committee of Cabinet, made up of Abbott, National Party leader Warren Truss, Foreign Affairs Minister Julie Bishop, Federal Treasurer Joe Hockey, Attorney General George Brandis, Defense Minister David Johnston and Immigration Minister Scott Morrison, will meet in Canberra on Friday morning to discuss the incident. Abbott told Fairfax Radio 3AW on Friday that if the plane had been shot down by a missile supplied by Russia, then "Russia bears a heavy share of responsibility" for the disaster.

Coalition frontbencher Christopher Pyne said the fate of MH17 appeared to be a "random event" amid the ongoing separatist dispute in the Ukraine.

"There's no doubt the plane was shot out of the sky," he told the Nine Network on Friday morning.

Opposition Leader Bill Shorten and foreign affairs spokeswoman Tanya Plibersek said the crash was devastating news.

"Our thoughts and deepest sympathies are with the family and friends of those on board, including at least 23 Australians," they said in a statement.

"The Labor opposition will do all it can to support the government at this terrible time."

Abbott said earlier on Friday on ABC radio that if the plane was shot down the perpetrators "should be swiftly brought to justice."

But Abbott has refused to discuss the possibility of Russia being banned from attending the G20 summit later this year while the circumstances that led to the tragedy are confirmed.

"The G20 is an economic gathering," he said. "It's not a security gathering. That doesn't mean that security issues are never discussed.

"But the principal purpose of the G20 is to try to ensure the world's largest and most representative economies are working constructively and, where possible, cooperatively for the benefit of all the world's citizens."