The Druzhba pipeline has become one of the most politically charged pieces of infrastructure in Europe since a Russian drone strike damaged the pipeline in western Ukraine and stopped Russian oil deliveries to Hungary and Slovakia.
Hungarian oil group MOL said on Wednesday that Ukraine had informed it that deliveries of Russian crude had resumed through the pipeline.
"MOL expects the first crude oil shipments following the restart of the Ukrainian section of the pipeline system to arrive in Hungary and Slovakia by tomorrow at the latest," it said in a statement.
Pumping began in the morning local time, an industry source said, asking not to be named because they were not authorised to speak publicly. Shortly afterwards, EU ambassadors meeting in Brussels approved the 90-billion-euro ($A148 billion) loan.
The European Union's 27 member states are expected to formally sign off on it by Thursday afternoon.
Ukrainian President Volodymyr Zelenskiy said the EU's decision was "the right signal under the current circumstances".
Writing on X, Zelenskiy said that incentives for Russia to end its war in Ukraine "can arise only when both support for Ukraine and pressure on Russia are sufficient".
The EU had agreed in principle to the loan last year to maintain Ukraine's liquidity through 2026 and 2027 but Hungary's Prime Minister Viktor Orban and the Slovak government had blocked it, accusing Ukraine of delaying repairs to the pipeline, which Kyiv denied.
Both Hungary and Slovakia are heavily dependent on Russian oil and Orban has consistently shown support for Russia.
Ukraine's prospects of receiving the loan had already improved when Orban lost Hungary's parliamentary election on April 12. The leader of the winning party, Peter Magyar, said he would no longer block the EU funds for Kyiv, though he is not expected to take power until next month.
The capacity of Druzhba, which in Russian means friendship, is 1.2 million to 1.4 million barrels of oil a day, with the possibility to increase to up to two million barrels a day. However, flows fell to a small fraction of that as a result of Western sanctions as well as repeated disruptions from drone attacks.
Separately, Germany confirmed that no Kazakh crude would reach its PCK Schwedt refinery - one of the country's largest - from May, after industry sources said on Tuesday that Russia was set to stop Kazakhstan's oil exports via the Druzhba pipeline.
