German Defence Minister Boris Pistorius arrived in Kiev for an unannounced visit to reaffirm Berlin's backing for Ukraine in its fight against Russia's troops.

Pistorius arrived on Tuesday by train and is due to hold talks with his Ukrainian counterpart as well as President Volodymyr Zelenskyy.

Berlin, the second-biggest supplier of military assistance to Kiev after the United States, is seeking to offer reassurances after the shift in focus to the Middle East war prompted concerns about waning support for Ukraine.

The visit comes after an increase in Russian air attacks on Ukraine and as Kiev braces for an expected rise in strikes on the country's energy facilities over the coming winter months.