Tokmak is quickly becoming a "lynchpin" of Russia's second main line of defence in southern Zaporizhzhia, according to British military intelligence.
Russia is deploying additional checkpoints, "hedgehog" anti-tank defences and digging new trenches in the area, said the UK defence ministry.
Ukraine has made slow but steady progress in the southern region, recapturing the town of Robotyne and making moves towards Verbove.
"Improvements to [Tokmaks'] defences likely indicates Russia's growing concern about Ukrainian tactical penetrations of the first main defensive line to the north," said the UK defence ministry.
The town is next on Ukraine's potential route to the coast, with the army seeking to split Russian forces in two at the Sea of Azov.
Hedgehog defences consist of metal girders forming several "X"-style shapes that are used to block tanks from advancing.
