Transneft's Druzhba pipeline was shut down at Poland's border with Belarus, Polish officials said.
The European Commission said the cuts posed no immediate risk to European supplies but it was seeking an urgent explanation from Belarus and Russia.
Exports were halted after Belarus began legal action against Russia for failure to pay a new oil shipment tax.
Transneft later said it had been forced to cut off supplies through the Druzhba pipeline after Belarus began siphoning off oil as payment in kind for the duties.
The Russian firm has so far refused to pay the oil export taxes as it claims the charges are illegal.
Transneft said it had been forced to cut of supplies through Belarus
The European Commission confirmed the supplies through the Polish part of the pipeline had been cut, adding it was investigating whether the move would have an impact on the other branch of the pipeline that goes to Slovakia and south-east Europe.
"I have also contacted our Russian and Belarusian authorities calling on them to provide an urgent and detailed explanation of the causes of this disruption," said European Energy Commissioner Andris Piebalgs.
Commission officials plan to look at whether European Union members will have to draw on strategic stockpiles to ensure their supplies, he added.
Poland's deputy economy minister Piotr Nalmski said his country had enough oil reserves for 80 days, but he attacked the decision to halt exports.
"This shows us once again that arguments among various countries of the former Soviet Union, between suppliers and transit countries, mean that these deliveries are unreliable," he told the BBC.
Meanwhile, German economy minister Michael Glos said he viewed the pipeline closure with concern and called on Russia and Belarus to meet their energy transit and delivery responsibilities "as soon as possible".
Officials in Minsk declined to comment on the situation, saying all questions should be directed to Transneft.
The disruption is a factor driving oil prices towards $57-a-barrel
The decision to shut down the Druzhba pipeline is the latest twist in an energy row between Belarus and Moscow which began when Russian energy giant Gazprom forced Belarus to accept a huge increase in the price of Russian gas.
Last week Belarus said it would charge Russia $45 (£23) per tonne of oil that passed through its country.
News of the disruption to supplies was a key factor helping to drive oil prices through the $57-a-barrel barrier after falling to around the $55 level last week.
US light sweet crude rose 89 cents to $57.20 in New York trade, while in the UK Brent crude stood at $56.71 - up $1.08.
News that Saudi Arabia, the world's largest oil producer, planned to cut output by 158,000 barrels a day also helped to drive crude prices higher.