Russia's President Dmitry Medvedev has ordered the state-controlled gas monopoly to cut gas supplies to ex-Soviet neighbour Belarus over its Russian natural gas debt.
Mr Medvedev's order follows his warning to Belarus last week to start paying off its 200 million US dollar (£134 million) debt or face cuts. Belarus challenged the Russian claim and refused to pay.
Mr Medvedev ordered the head of gas giant Gazprom to implement cuts on Monday.
Belarus' economy has been struggling to cope with rising Russian natural gas prices and excise hikes on Russian oil imports.
Belarus President Alexander Lukashenko said Russia should provide it with cheap oil and gas as part of the customs union deal to come into force on July 1, but Moscow rejected this demand.