Russian troops are making steady progress in Ukraine’s east on the back of more concentrated artillery and air power, now controlling almost all of the Luhansk region and threatening to encircle thousands of Ukraine’s most experienced troops.

That is sparking fears that Russia could be poised for a bigger breakthrough, and leading to increasingly panicked calls from Kyiv for even more powerful offensive weapons.

UK Prime Minister Boris Johnson described the Russian gains as “slow, but I’m afraid palpable, progress”. He also backed Ukrainian demands for supplies of longer range multiple launch rocket, or MLRS, systems as “where the world needs to go”...