Russia has been advancing steadily in Ukraine's eastern region of Donetsk and further south, along sectors of the front that Kyiv has struggled to hold, citing manpower and ammunition shortages.

"The enemy is well aware that as a result of the gradual receipt of a significant amount of weapons and military equipment from our partners, and the arrival of the first F-16s, which will strengthen our air defence, time will play in our favour and its chances of success will decrease," said Ukrainian army chief Oleksandr Syrsky.

"Therefore, the command of Russia's troops is currently making every effort to increase the intensity and expand the geography of hostilities in order to maximise the depletion of our troops, disrupt the training of reserves and prevent the transition to active offensive actions," he added in a statement on social media.

Ukrainian President Volodymyr Zelensky said this weekend at a peace summit in Switzerland that current levels of military aid from abroad were still insufficient and that deliveries were arriving late.

He has appealed to allies to speed up deliveries of dozens of promised F-16 fighter jets and called on Western countries to send more air defence batteries.

Syrsky said Russian forces were concentrating their firepower on the Donetsk region -- which the Kremlin says is part of Russia -- particularly on the Pokrovsk front.

Moscow's forces there are closing in on a key transit artery and supply route linking civilian hubs in the north of the industrial territory to towns further south.

His battlefield assessment came just hours after regional authorities in the southern region of Kherson, which is partially occupied by Russia, said a 50-year-old civilian had been killed in a drone attack.

Russian forces also struck "civilian infrastructure" in the eastern Poltava region, its governor said, without elaborating.

Governor Filip Pronin said nine people had been wounded in the attack and several multiple-storey buildings were damaged.