DFAT aware captured Australian soldier Oscar Jenkins was 'missing ...

Australian authorities were alerted to the "disappearance" of Melbourne man Oscar Jenkins at least three months before a hostage video emerged of the captured soldier being interrogated by Russian forces in eastern Ukraine.

The ABC has learnt that the Department of Foreign Affairs and Trade (DFAT) was first made aware of concerns for the welfare of the 32-year-old former teacher as far back as August, but diplomats still cannot confirm his current condition and location.

An American soldier who previously served alongside Mr Jenkins in Ukraine's armed forces has also described his anguish at recently learning his friend had been captured by the enemy and his urge to want to run into battle to save him.

"First thought I wanted to do was get the f*** up, find out how to f***ing get to him and be the f***ng person to drag his ass out of prison camp," the US foreign fighter has told the ABC's AM program.

The American national, who asked to be identified only by his call sign "Forrest" because he was still serving in Ukraine, said his Australian comrade was a selfless soldier, willing to face danger and often gave away money to those who needed it.

"He was ready to go [to the] front line with no armour, no weapon, and just there to kill Russians and keep Ukrainians safe, like, he was in it for Ukraine," Forrest said.

"He was very patriotic and he was the hell of a damn good soldier too … everything he did was to keep f***ing Ukrainians safe," he added.

American "Forrest" with Melbourne man Oscar Jenkins. (Supplied)

Forrest, who only learnt of his Australian friend's capture after the publication of the Russian military's hostage video, told the ABC he was "just worried sick about him".

A diplomatic source familiar with Mr Jenkins's case says the Australian government does not doubt the authenticity of the hostage video that emerged over the weekend, but says authorities are yet to determine when precisely it was recorded.

Mr Jenkins's family in Melbourne declined to comment about his situation when contacted by the ABC on Monday but were receiving consular support from DFAT officials.

On Monday afternoon, Russia's ambassador to Australia, Alexey Pavlovsky, was hauled into a meeting at DFAT headquarters in Canberra for about half an hour but declined to comment as he entered and left the building.

In a statement, Acting Foreign Minister Mark Dreyfus said the Australian government was "making representations to the Russian government" about the case.

"We urge the Russian government to fully adhere to its obligations under international humanitarian law, including with respect to prisoners of war," Mr Dreyfus said.

"I reiterate the government's clear advice to all Australians — do not travel to Ukraine."

Australian pro-Kremlin propagandist Simeon Boikov — known by his online moniker Aussie Cossack — has reposted the apparent hostage video, stating that he should be part of a prisoner swap deal.

The outspoken social media figure is the leader of the Australian Cossacks, which styles itself as a military unit, and is holed up in the Russian consulate in Sydney to avoid an arrest warrant for an alleged assault.

Last week, prosecutors also revealed that a "significant volume" of foreign language material had allegedly been found on the devices of two accused Russian spies, Kira Korolev and her husband Igor, leading to delays in their espionage case.

At least seven Australians are believed to have died fighting in Ukraine since Russia's invasion began in 2022, but Oscar Jenkins is believed to be the first to be captured and held as a prisoner of war.