The changes replace the old system of indefinite military service, whereby mobilised men were expected to fight until the end of Russia's invasion, and come after years of manpower shortages and complaints of low troop morale.

When Russia launched its full-scale invasion in 2022, military recruitment offices were overflowing with volunteers seeking to defend the country.

Now, almost all new arrivals are conscripts who did not sign up of their own free will, while the army has been plagued by mass desertions, according to Ukrainian media reports.

"The new contracts provide clear service terms, guaranteed deferments after completion of service, a new system of financial support, and greater predictability and control over one's own service," Ukraine's defence minister Mykhailo Fedorov said in a social media post.

From Monday, active servicemen as well as citizens will be able to sign new contracts ranging from six months to 24 months, he said.

"After completing service, a guaranteed deferment is provided that takes into account previous service duration and combat experience," he added.

It was not immediately clear how the system would work in practice.

The new contracts also offer lucrative salaries to frontline assault troops, from an average of 300,000 Ukrainian hryvnia a month ($6,700) to a maximum of 460,000 ($10,300) for the riskiest deployments.