President Volodymyr Zelensky of Ukraine has signed into law three measures aimed at replenishing the ranks of his country’s exhausted and battered army, including the politically poisonous step of lowering the age when men become eligible for mobilization, and eliminating some medical exemptions.

Parliament passed the legislation lowering the draft eligibility age to 25, from 27, last May, but Mr. Zelensky had delayed signing it in hopes that it would not be needed. He relented on Tuesday and signed the measure, along with laws eliminating a category of medical exemption known as “partially eligible” and creating an electronic database of men in Ukraine, starting at age 17, to crack down on draft dodgers.

“It is a very unpopular decision, and that is why Zelensky held it without signing,” said Volodymyr Ariev, a lawmaker in Parliament who is in the opposition European Solidarity party. “Now he has no choice.”

Russia’s forces have been on the offensive along the front line, and Ukrainian generals have warned of a broader attack in the spring or summer, even as Ukraine’s army runs low on ammunition and many soldiers have been on continual combat duty for two years.

Ukraine’s army of about one million soldiers is fighting the largest war in Europe since World War II, waged in muddy trenches or the ruins of cities in urban combat. Casualty rates are high. Most men who wanted to volunteer for the military have already done so, and small anti-draft protests had broken out before the new laws were passed.