SYDNEY (Reuters) - Australian Prime Minister Tony Abbott said if the downing of a Malaysian airliner over eastern Ukraine was a criminal act, then "it is an unspeakable crime".

The Australian government said in a statement it had grave concerns that a number of Australians might have been on the plane, which crashed on Thursday, killing all 295 people on board.

The European head of Malaysia Airlines said at least 27 Australian were on the flight.