European Union negotiators failed to reach a deal on an embargo of Russian oil but will continue attempts to reach an accord before the beginning of an EU summit starting on May 30, an EU official said.

Ukrainian President Volodymyr Zelenskiy is scheduled to address the summit through a video link to press efforts to intensify sanctions against Moscow to punish it for its invasion of his country.

EU countries are discussing a proposal in which member nations would accept a ban on Russian oil delivered to the EU by sea by the end of the year.

Exemptions would be given to countries -- Hungary, Slovakia, and the Czech Republic -- receiving oil via the 4,000-kilometer long Russian Druzhba pipeline.

Bloomberg, citing sources familiar with the talks, reported that Hungary was so far refusing to back the compromise.

The report said Budapest had previously suggested it would give its support based on the exemption to pipeline deliveries. But it said Budapest was now demanding further concessions.

Hungary, led by right-wing Prime Minister Viktor Orban, is dependent for 65 percent of its oil needs on crude supplied through the Druzhba pipeline, which runs from Russia to Eastern and Central Europe.

In 2021, Russia shipped some 720,000 barrels a day of crude oil to European refineries through its main pipeline to the region, compared with 1.57 million barrels a day through seaborne delivery from its Baltic, Black Sea, and Arctic ports.

The bulk of the pipeline deliveries go to Germany and Poland, but those countries have indicated they will attempt to reduce their reliance on Russian supplies regardless of any actions by the EU.

Russia last year received some $86 billion from its total oil exports.

German Economy Minister Robert Habeck on May 29 expressed concerns that EU unity was "starting to crumble" as the bloc prepares to meet discuss an oil embargo and further efforts to cut reliance on Russian energy.

EU leaders will meet on May 30-31 to discuss possible new sanctions against Moscow, including an oil embargo, to punish it for its full-scale invasion of Ukraine.

"After Russia's attack on Ukraine, we saw what can happen when Europe stands united. With a view to the summit tomorrow, let's hope it continues like this. But it is already starting to crumble and crumble again," Habeck told a news conference.