Australian officials are desperately trying to confirm whether Melbourne teacher Oscar Jenkins has been killed by Russian forces after being captured while fighting for Ukraine.
The Australian Federation of Ukrainian Organisations said it was shocked at the reports and called for a "full force" response if confirmed.
The government should downgrade diplomatic relations with Russia if Mr Jenkins had been executed, federation co-chair Kateryna Argyrou said.
"The Russian ambassador should be expelled and ordered to leave on the first available flight, as should all Russian diplomats and spies," she said.
"The Russian embassy should be closed, along with the Russian consulate in Sydney."
The strongest possible action will be taken if Oscar Jenkins has been harmed, Anthony Albanese says. (Lukas Coch/AAP PHOTOS)
Prime Minister Anthony Albanese has vowed to take "the strongest action possible" if Russia has executed him.
"We call upon Russia to immediately confirm Oscar Jenkins' status, we remain gravely concerned," he told reporters in Tasmania on Wednesday.
"We'll await the facts to come out but if there has been any harm caused to Oscar Jenkins, that's absolutely reprehensible and the Australian government will take the strongest action possible."
The 32-year-old was captured and shown in social media videos visibly bruised and shaken while being interrogated and beaten by a Russian-speaking captor.
The Russian ambassador to Australia has been hauled in over the issue as Australia seeks more information.
The federal government has so far resisted expelling the ambassador, arguing it is better to keep channels of communication open.
"This has been a very difficult relationship for many years, including the downing of MH17 in 2014 as well as the illegal and immoral invasion of Ukraine," Foreign Minister Penny Wong said.
"Australia has maintained diplomatic relations with Russia through that period under different governments.
"However, we will consider all options once we have ascertained the facts and once we can verify what has actually occurred."
Senator Wong said Russia was expected to comply with international humanitarian law.
The opposition is demanding the expulsion of the Russian ambassador if the reports are true.
Liberal senator Simon Birmingham said if Russia had undertaken an extrajudicial execution of a captured Australian citizen the Albanese government should respond in the strongest possible terms.
"Nothing less than the recalling of Australia's ambassador to Russia and expulsion of Russia's ambassador would be sufficient in such circumstances," he said.
Australian Oscar Jenkins was fighting for Ukraine before being captured by Russian forces. (HANDOUT/'SLADKOV+' TELEGRAM ACCOUNT)
Kyiv has "great sympathy" for fighters like Mr Jenkins and sent its respects to his family, Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelenskiy, told ABC News.
"Russia has a deep-seated hatred of foreigners like Mr Jenkins, people who have made the responsible choice to stand up for the rule of law and freedom by supporting Ukraine," he said, according to a translation.
Russian soldiers had no understanding or interest in the laws of war, retired major general and military strategist Gus McLachlan said.
"The Russian soldiers on the front line are brutal, many have been released from prisons," he said.
"It's highly likely Oscar was badly beaten at the time of his capture and he may simply have been neglected to a point where he died of either wounds or exposure or malnutrition."
Mr McLachlan said he thought it would have been in Russia's interest to keep the Australian alive and use him as part of "information warfare".
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