Explainer: What are tactical nuclear weapons, what is Russia’s policy?

Kyiv and its Western allies fear tactical nuclear weapons could be used in battle after Putin and others warned Russia was prepared to use all its vast arsenal in defence.

Russia has a deal to station tactical nuclear weapons on the territory of neighbouring Belarus, President Vladimir Putin said on Saturday, adding this will not violate non-proliferation agreements. What are these weapons and what is Russia's policy on them: