A definition of marriage as between a man and a woman is among several amendments to the Russian constitution proposed by president Vladimir Putin.

The State Duma, Russia's lower parliamentary house, approved the constitutional reform bill in January. Putin's amendments will be included in a second reading next week.

A public vote on the constitutional amendments will take place April 22, but will first have to undergo approval from the Constitutional Court.

Neither same-sex marriage nor civil unions are legal in Russia.

Two weeks ago, Putin spoke on same-sex marriage, saying Russia would not legalize the practice while he is in power.

"As far as 'parent number 1' and 'parent number 2' goes, I've already spoken publicly about this and I'll repeat it again: As long as I'm president this will not happen. There will be dad and mum," Putin said.

Critics charge that the changes to the constitution are a means for Putin to maintain power, whose fourth term as president ends in 2024.

Other proposed amendments would include a reference Russians' "faith in God", and one on "historical truth" that would preserve "the great achievement of the people in their defence of the Fatherland".

The historical truth amendment would emphasize the Soviet Union's role in World War II, during which some 27 million Soviets lost their lives fighting Nazi Germany.

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Another amendment would forbid the turning over of any Russian territory, which could strengthen Russia's claims to Crimea, a Ukrainian region it annexed in 2014, and to the Kuril Islands, an archipelago it administers but some of which is claimed by Japan.