Malcolm Turnbull has wished "American hero" John McCain a speedy recovery after the United States senator was diagnosed with a brain tumour.
The diagnosis came after the former presidential candidate had surgery to remove a blood clot from above his left eye.
"Lots of love and best wishes for a swift recovery," the prime minister tweeted on Thursday.
"An American hero & great friend of Australia."
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Australia's ambassador to the US Joe Hockey said it was terrible news.
"Australia's thoughts and prayers are with him for a speedy recovery," he wrote on Twitter.
Terrible news about our great friend @SenJohnMcCain.
Australia's thoughts and prayers are with him for a speedy recovery.
