Ukrainian President Volodymyr Zelensky says Russia could increase its attacks in Ukraine's north east following its recent gains near the city of Kharkiv, Ednews report via BBC.
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."
The US recently approved a new $61bn (£48bn) aid package for Ukraine, which includes military support.
Russian forces recently began a summer offensive, grabbing a number of villages on Ukraine's north-eastern frontier near Kharkiv - the country's second largest city.
Kharkiv's regional governor Oleg Synegubov said on Saturday that nearly 10,000 people have been forced to leave in just over a week.
"The situation is controlled by defenders of Ukraine," he added.
Polish Prime Minster Donald Tusk, meanwhile, has announced the country will spend over €2.3bn (£1.9bn) to fortify its eastern border against "potential enemies".
The border includes Russia, Belarus and Ukraine.
