(Updates with more details, quotes)
By Jane Wardell
SYDNEY, July 18 (Reuters) - Australian Prime Minister Tony Abbott said that if the downing of a Malaysian airliner over eastern Ukraine was a deliberate act, then "it is an unspeakable crime" and the perpetrators must be swiftly brought to justice.
Abbott said on Friday that he believed at least 23 Australians were on Malaysia Airlines Flight MH17 from Amsterdam to Kuala Lumpur.
Abbott was speaking shortly after the European head of Malaysia Airlines said at least 27 Australians were among the 295 passengers on the flight.
If the numbers are confirmed, Australians would represent the second-largest national grouping, behind the Dutch, on the plane that crashed on Thursday.
"It is a very, very sad time, made worse by reports that it might be a crime rather than an accident," Abbott said.
Ukrainian authorities have accused "terrorists" - militants fighting to unite eastern Ukraine with Russia - of shooting down the Boeing 777-200. The rebels denied responsibility.
U.S. Vice President Joe Biden said the downing of the airliner apparently was "not an accident" and that it was "blown out of the sky".
"If it does turn out that this aircraft was brought down by a surface to air missile, there is no doubt that this would be, under those circumstances, an unspeakable crime and the perpetrators should swiftly be brought to justice," Abbott said.
Abbott declined to be drawn on the reason for the crash, saying all reports remained unconfirmed.
"We just have all sorts of reports and claims flying around," he said. "That's all we have at the moment, reports and claims."
Abbott said he planned to speak with Malaysian Prime Minister Najib Razak later on Friday and that Australia would deploy staff to Kiev to assist in an investigation.
Asked if he would still welcome Russian President Vladimir Putin to Australia for the G20 Leaders Summit later this year if Russia were proved to be behind the accident, Abbott hinted there may be repercussions.
"That's a fair question, let's just wait and see exactly what turns out to have happened here," he said.
"But obviously, it is the clear and settled decision of the Australian government that larger countries should not bully smaller ones, that countries should not aid people who are in rebellion against their own government and that international disputes should be settled peacefully in accordance with international law."