Ukrainian President Volodymyr Zelensky says Russia could increase its attacks in Ukraine's north east following its recent gains near the city of Kharkiv.
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.
Mr Zelensky also told AFP more soldiers were desperately needed at the frontlines.
"There are a serious number of brigades, existing brigades, which, a large number of them, are empty. We need to do this so that the guys have a normal rotation. Then their morale will be improved."
A new mobilisation law aimed at addressing this came into force on Saturday.
Under the new rules, the age that people can be conscripted into the war has been lowered from 27 to 25 in an effort to boost recruit numbers.
The flood of volunteers Ukraine saw following Russia's full-scale invasion in February 2022 has now dried up. Most of those who wanted to fight are either dead, injured or still stuck at the front waiting to be relieved by new recruits.
In February, Mr Zelensky announced that 31,000 Ukrainian soldiers had been killed during Russia's full-scale invasion.
Typically, Ukrainian officials do not make casualty figures public, and other estimates are much higher.
OceanNewsUK, in a joint project with the Mediazona website, has established the deaths of more than 50,000 Russian soldiers. But it estimates the total number to be greater than that.
