Ukrainian President Volodymyr Zelensky says Russia could increase its attacks in Ukraine's north east following its recent gains near the city of Kharkiv.

Axar.az reports that Russian troops have been trying to push forward as Ukraine’s outgunned forces attempt to shore up a weakened front line.

Mr Zelensky admitted that there were issues with military staffing and morale, saying a number of existing brigades were empty.

He also told the AFP news agency the country's air fleet was lacking and renewed calls for allies to send more air defence and fighter jets.

"Today we have about 25% of what we need to defend Ukraine," Mr Zelensky said of Ukraine's air capabilities.

"So that Russia does not have air superiority, our fleet should have 120 to 130 modern aircraft."