Ukrainian President Volodymyr Zelensky has signed into law a bill lowering the military mobilisation age by two years from 27 to 25.
Kyiv has faced heavy losses on the battlefield after two years of war, whereas Russia has benefited from a sizeable advantage in manpower.
The move will allow Ukraine to call up more people to replenish its reserves, after volunteer numbers dropped.
Mr Zelensky said in December that 500,000 more soldiers were needed.
Ukraine’s parliament approved the bill in May 2023, but it did not come into force because Mr Zelensky did not sign it.
In a separate move, Nato foreign ministers in Brussels were due to consider plans for a €100bn (£86bn) five-year fund to provide long-term military support for Ukraine.
Asked if the figure was correct, Nato Secretary General Jens Stoltenberg said the framework was yet to be decided but Nato allies agreed that Ukraine needed more money and for many years.
Russia was “pushing” on the battlefield in Ukraine and trying to win the war by “waiting us out”, he said.
It was not immediately clear what prompted President Zelensky to approve a law to lower the age of mobilisation, although he has previously warned of plans Russia may have to launch a spring or summer offensive this year.
His decision could signify Ukraine’s effort to build up strong defensive fortifications in preparation for a Russian offensive.
Ukrainian forces had hoped to take back swathes of territory gained by Russia as well as cutting off Russian supply lines to Crimea.
But lacking air superiority and faced with formidable Russian defences, Kyiv’s counter-offensive ground to a halt at the start of winter and there are fears that Russian forces could outgun Ukraine.
