Ukraine's parliament on Wednesday passed a bill that would enable some prisoners to fight in the armed forces, as the nation faces a critical shortage of manpower on the front lines.

Long opposed to the measure and having criticised Moscow's mobilisation of prisoners to fill its ranks, Kyiv has recently U-turned amid fresh Russian advances on the battlefield.

The legislation would need to be signed by the chairperson of parliament -- the Verkhovna Rada -- and President Volodymyr Zelensky before coming into force.

"The parliament has voted 'yes'," MP Olena Shuliak, head of Zelensky's party, said in a Facebook post.

"The draft law opens the possibility for certain categories of prisoners who expressed a desire to defend their country to join the Defence Forces," she said.

Mobilisation would be voluntary and only open to certain categories of prisoners.

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, Shuliak said.

Only prisoners with under three years left on their sentence can apply, she added. Those prisoners who are mobilised are granted parole rather than a pardon.

 'Redemption by blood'

The organisation Protection for Prisoners of Ukraine, which had lobbied for a measure allowing prisoners to fight, was disappointed with the adopted text.

"We support the idea behind the law... but the text that was passed is discriminatory," said NGO head Oleg Tsvily.

"They got rid of leave for (fighting) prisoners and we don't know if they're meant to fight until the war ends -- which could mean longer than their sentence," he explained.

Tsvily also feared the creation of "special units" for mobilised soldiers would lead to abuse against prisoners.

"It's like in Russia, redemption by blood... Anyone willing to fight will be put in one unit and commanded like meat," he said.

He was referring to reported practices from the Wagner mercenary group of sending waves of convicts into assaults likened to "meat grinders".

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.

The practice was spearheaded by Yevgeny Prigozhin, who was filmed touring Russian prisoners to recruit foot soldiers for his Wagner paramilitary group.

More than two years into the war, Kyiv is grappling with how to recruit enough soldiers to repel an intensification of Russian attacks on the front lines.

It has recently toughened measures against draft dodgers, as well as lowering the age at which men can be drafted from 27 to 25.