Ukrainian president Volodymyr Zelensky told his end-of-year news conference on Tuesday it was a "highly sensitive" issue that the military and government were discussing before sending to parliament.
Ukraine has been at war with Russian troops since February 2022.
Both sides treat their casualty numbers as a state secret, but US officials estimate hundreds of thousands have been killed and wounded.
Ukraine's troop numbers are not known, but it has said in the past it has around one million people under arms.
Russia has been expanding its army during the war and said on Tuesday it planned to boost its ranks to 1.5 million service members.
Mr Zelensky said Ukraine's military had proposed mobilising 450,000 to 500,000 more Ukrainians into its armed forces.
"I said I would need more arguments to support this move," he said.
"Because first of all, it's a question of people, secondly, it's a question of fairness, it's a question of defence capability, and it's a question of finances."
Ukraine, which initially saw tens of thousands of volunteer fighters queuing up to defend their country from Russia's invasion, is now trying to conscript more men to replace those currently at the front.
There have been discussions behind closed doors for weeks on how to improve the draft process. Some Ukrainians have reacted angrily to social media videos showing draft officers handing out call-up papers at gyms and resorts.
