Australia has vowed it will take the “strongest action possible” if Russia has harmed one of its nationals captured fighting for Ukraine, after reports emerged the prisoner of war may have been killed.

Australia's Prime Minister Anthony Albanese has vowed to take decisive measures if harm has been inflicted on Oscar Jenkins, a Melbourne school teacher captured while fighting for Ukraine. Disturbing video footage from December shows Jenkins being interrogated by Russian forces.

There are fears an Australian man captured by Russian forces while fighting for Ukraine has been executed. Oscar Jenkins, a 32-year-old teacher from Melbourne, was reportedly captured in the Donbas region.

Moscow launched more than 40 missiles, and at least 30 were shot down, said Ukraine’s President Volodymyr Zelenskyy, adding about 70 strike drones were involved in the attack. It remains unclear how many were intercepted.

Russia’s military on Tuesday said it would retaliate against Ukraine after Kyiv attacked Russian regions by firing six US-made ATACMS ballistic missiles, six UK-made Storm Shadow cruise missiles and launching one of the biggest drone attacks to date.

Ukraine introduced emergency power cuts in six regions amid what authorities described as a "massive" Russian missile strike, the national grid operator said on Wednesday.