Australia said Wednesday that "all options are open" in its response to the reported killing of one of its citizens after his capture by Russian forces in Ukraine.
Foreign Minister Penny Wong said the government was working "very hard" to confirm a report that Australian Oscar Jenkins had been killed.
"We hold grave concerns for his welfare," she said.
Australia's Channel Seven said his body had been found but had not yet been officially identified, citing unnamed sources.
Video emerged online in December of Jenkins -- who reportedly fought with Ukrainian forces -- being questioned and slapped in the face by a man speaking Russian.
Wong told broadcaster ABC that Russia's ambassador to Australia had been called in this week "at my direction" over the matter.
Asked how Australia would react if the killing was confirmed, and whether the Russian ambassador might be expelled, she said: "I want to be clear, all options are on the table."
Prime Minister Anthony Albanese said the government was "greatly concerned".
"We will await the facts to come out, but if there has been any harm caused to Oscar Jenkins that is absolutely reprehensible, the Australian government will take the strongest action possible," he told reporters.
The Kremlin said Wednesday it was "not aware" if the Australians had raised the specific case with Russia's foreign ministry, but said foreigners fighting with Ukraine were "legal targets" for Russia's troops.
"We are well aware that foreign mercenaries are taking part in the conflict on the side of the Kyiv regime," Dmitry Peskov told reporters.
"They are fighting against our military with weapons in their hands. Accordingly, they become legal targets," he added.
Australia's foreign minister said Russia had an obligation to treat prisoners of war in line with international law, including humane treatment and the right to a fair trial.
Australian relations with Moscow have been "very difficult" for many years, Wong said.
She cited the "illegal and immoral" Russian invasion of Ukraine in February 2022, and the July 2014 downing of Malaysian Airlines flight MH17 by a Russian-made missile.
The MH17 crash over eastern Ukraine killed 298 people aboard, including 38 Australians.
"Australia has maintained diplomatic relations with Russia through that period under different governments," Wong said.
"However, we will consider all options once we have ascertained the facts and once we can verify what has actually occurred here with Mr. Jenkins."
Wong said her thoughts were with Jenkins' family.
"They have lived with the fear and uncertainty of a loved one in the middle of a foreign war for many months," she said.
"I know these reports will be devastating to them, and they are in my thoughts, and I'm sure the thoughts of many Australians."
© Agence France-Presse