Religious freedom may be a casualty of Russia's war in Ukraine, a Christian human rights group has warned.

There are fears that the persecution of Christians will increase if Russia widens its control over Ukraine.

A partner of Release International in neighbouring Poland said, "There is no doubt that wherever Putin has established his power the persecution of Christians has increased substantially." 

In parts of Ukraine already under Russian separatist control, many Protestant churches have been closed and some Christian books have been banned.

Donetsk and Luhansk in the disputed Donbas region have been under the control of pro-Russian separatists since 2014. 

Forum 18, a Christian persecution monitor in Norway, reports that in rebel-held regions the religious freedom of Protestants and churches not belonging to the Orthodox Moscow Patriarchate has been severely restricted.

In Luhansk, a Religion Law passed in 2018 required churches to re-register with the authorities effectively forcing churches to "apply for permission to exist", Forum 18 said.

Since then, the authorities have refused to register all Protestant, Baptist and Pentecostal congregations. 

In March 2019, Baptist Union churches were threatened with punishment if they did not suspend public worship. 

Raids have been carried out on places of worship and the congregations of unregistered churches have been denied access to their buildings or had the water, gas and electricity supply cut off. 

The Luhansk authorities have also banned 12 Christians books on an "extremist material" list, Forum 18 said. These include an 1820 edition of the Gospel of John, a Baptist hymnbook, and books by Charles Spurgeon and Billy Graham. 

Release CEO Paul Robinson said he was concerned that religious persecution could spread in Ukraine if Russia tightens its grip on the country.

"These are dark days, not only for the Ukrainian people, but for religious freedom. They need our prayers," he said. 

"Freedom of Christian worship has been severely curtailed in areas previously under Russian control. This is likely to get worse if Russia tightens its grip."