"On August 4, the delivery of Russian oil via the territory of Ukraine was halted," the oil pipeline operator said in a statement, adding that this had affected deliveries to Hungary, Slovakia and the Czech Republic through the southern leg of the Druzhba pipeline.

The company said the Ukrainian side stopped the oil transport "due to not receiving funds for these services".

However, deliveries to Poland and Germany via Belarus were continuing "as usual", Transneft added.

A spokesman for Slovak refinery Slovnaft confirmed that the transportation of oil through the Druzhba pipeline had been suspended for several days.

But he said that the company's Bratislava refinery was operating and supplying the market.

"According to our information, there were technical problems at the bank level in connection with the payment of transit fees from the Russian side," Anton Molnar said in a statement.

Linda Vaskovicova, head of the Slovak oil transporter Transpetrol CEO's office, told the local TASR newswire: "We can confirm that the transportation of oil through the Druzhba pipeline through the territory of the Slovak Republic is currently suspended."

She said that the authorities were "dealing with the situation."

Molnar also said that Slovnaft had initiated talks with Ukraine and Russia about the possibility of Hungarian refinery MOL and Slovnaft paying the transit fees, which would enable the resumption of oil supplies.

Since President Vladimir Putin sent troops to Ukraine, Washington and Brussels have pummelled Moscow with unprecedented sanctions, cutting Russia off from international financial institutions.

The EU has looked to reduce its dependence on Russian energy resources and has agreed to ban more than two-thirds of Russian oil imports.

The United States banned Russian oil and gas days after the start of Russia's military campaign on February 24.