The move echoes a policy in Russia, where tens of thousands of prisoners have been sent to Ukraine with the promise of amnesty and were killed in gruelling battles that produced few gains.

Ukraine is suffering critical ammunition and manpower shortages on the battlefield that have allowed Russian forces to advance on the eastern and northern front lines.

"This is more than 3,000 people. We predicted this before the adoption of this law," Deputy Minister of Justice Olena Vysotska said, referring to the number of prisoners who have submitted applications to join the military.

She said authorities had identified 20,000 eligible prisoners and that of them, 4,500 had "expressed interest" in joining. She added that the figure was likely to fluctuate.

Only prisoners with fewer than three years left on their sentence can apply. Mobilised prisoners are granted parole rather than a pardon.

Among those not eligible to serve include those found guilty of sexual violence, killing two or more people, serious corruption and former high-ranking officials.

Russia has recruited prisoners to serve on the front lines since the first days of its invasion, initially offering presidential pardons for six months' service.