Russia has accused Belarus of stealing oil from a major pipeline and has shut off oil exports to its western neighbour, halting supplies to Poland and Germany and threatening wider disruptions in central Europe.
Russia's pipeline monopoly Transneft said today it was forced to act because Belarus had been siphoning off oil illegally from the Druzhba pipeline system.
The oil supply cut was reminiscent of a stand-off last year between Russia and Ukraine that hit gas supplies to Europe.
The move escalates a tit-for-tat dispute between Russia and Belarus, which have imposed punitive oil levies on each other.
The European Union demanded an "urgent and detailed" explanation, a spokesman for Energy Commissioner Andris Piebalgs said.
Europe is heavily reliant on energy powerhouse Russia for its oil and gas and extremely vulnerable to Russian supply cuts.
"The Belarussian side began taking transit oil as payment in kind for a new duty it had illegally imposed," said Transneft vice-president Sergei Grigoryev.
"We therefore reduced transit supplies, equal to the amount being taken. We then reached the point where we had to stop supplies completely."