The controversial bill, which President Volodymyr Zelensky signed into law on Tuesday, is the most significant overhaul of Ukraine’s war bureaucracy since Russia’s full-scale invasion began in February 2022.

Men between 18 and 60 have been prohibited from leaving the country since the start of the war, but only those who were at least 27 were eligible to be drafted. Zelensky’s backers and some opponents said the bill had been delayed because it was unpopular.

The change comes as Ukraine’s defensive lines come under heavy pressure from massive Russian assaults. Front-line commanders say they are short of personnel and ammunition, as a supplemental aid package is stuck in Congress. Ukrainian officers say they are expecting Russia to mount a significant offensive around the start of the summer, which is aimed at expanding the nearly 20% of territory the invading forces already occupy.

It is unclear how many men will be added to the ranks under the new law. In December, Zelensky said the military had requested 500,000 more soldiers, but he has repeatedly questioned whether this total was feasible or necessary.

“We don’t need half a million,” Zelensky said at a press conference on Wednesday, adding that an audit of the armed forces found many soldiers who hadn’t been deployed to the front and could be sent there now. “As for the specific number, how many will be mobilized, I’m not ready to tell you yet. I can say that Russia is preparing to mobilize 300,000 troops for June 1.”

Col. Gen. Oleksandr Syrskiy , who took over as commander in chief of Ukraine’s armed forces in February, also said in a recent interview with Ukrainian media that he would not need 500,000 additional troops, but didn’t provide an estimate of how many he did need. He also said he would give priority to resting troops who have been fighting at the front, some of whom have hardly had a break for two years.

“People are not robots,” Syrskiy said. “They are exhausted, physically and psychologically, especially in combat conditions. For example, those who came to military recruitment centers in February 2022—these people need rest and treatment.”

Zelensky also signed two additional bills on Tuesday that are designed to make it harder for men to avoid conscription.

One will establish an electronic database of all men who are eligible for the draft. Many men have avoided registering at recruitment houses, and the bill allows the Defense Ministry to pull phone numbers, addresses and other information from other Ukrainian government agencies.

The third bill requires everyone who has received a medical exemption from military service to undergo a new physical examination within nine months.

Though Ukraine has an unusually high age of conscription—in many Western countries, including the U.S., men can be drafted at 18—lowering it has proved unpopular.

Destruction in the front-line town of Toretsk. PHOTO: SERHII KOROVAYNY FOR THE WALL STREET JOURNAL

Most Ukrainians who want to fight have already volunteered. Meanwhile, almost everyone in the country knows someone who had been injured or killed in action over the past two years. Zelensky said in February that 31,000 Ukrainian soldiers had been killed in action, but military analysts say the true total is likely far higher.

As a result, a shortage of troops has slowly grown, while Russia, with a population four times as large as Ukraine’s, has been adding around 30,000 new troops to its available forces every month, according to Ukrainian officials. Ukraine is also outgunned, with deliveries of additional ammunition and other weapons from the U.S. held up in Congress.

Russia, meanwhile, has stepped up airstrikes against Ukraine that have targeted energy infrastructure and hit civilians. A Russian missile strike Tuesday on the Ukrainian city of Dnipro injured at least 18 people.

Ukraine has targeted Russian oil infrastructure in recent weeks using long-range explosive drones. On Tuesday, Ukrainian drones hit one of Russia’s largest oil refineries in Tatarstan, 500 miles east of Moscow.

“We continue to work systematically to ensure that Russia has less and less opportunities to finance the war of aggression against Ukraine,” a Ukrainian official said.

Write to Ian Lovett at