Russia halted gas supplies to Bulgaria and Poland on Wednesday for rejecting its demand to pay in roubles, taking direct aim at European economies in its toughest retaliation so far against international sanctions over the war in Ukraine.

The step was denounced by European leaders as "blackmail", and comes as European countries have joined the United States in ramping up arms shipments to help Ukraine fend off a new Russian assault in the east.

Ukraine reported on Wednesday that Russian troops had made gains in several villages there. Russia reported a number of blasts on its side of the border, including a blaze at an arms depot, which Kyiv called "karma".

Moscow says the gas cut-off is to...