Hungarian Prime Minister Viktor Orban has criticised European Commission head Ursula von der Leyen for "attacking" EU unity with a proposed Russian oil ban, saying it crosses a red line.

"The European Commission president, intentionally or unintentionally, has attacked the European unity that had been worked out," Mr Orban said on state radio.

"From the first moment we made clear that there will be a red line, that is the energy embargo, they have crossed this red line."

Meanwhile, two sources have told Reuters that the European Commission has amended a proposal for an embargo on Russian oil to extend the period before it takes effect for Hungary, Slovakia and the Czech Republic.

Under the tweaked proposal, Hungary and Slovakia will continue to be able to buy Russian oil from pipelines until the end of 2024, whereas the Czech Republic could continue until June 2024, provided that it does not get oil via a pipeline from southern Europe earlier, the sources said.

Bulgaria had also asked for exemptions, if others obtained them, but was not offered concessions on deadlines, "because they don't have a real point," one official said.

The other three countries who were granted more leeway "have an objective problem," the official added.

One of the sources said that the extended deadlines were calculated on the likely construction times for pipeline upgrades.

The official said Hungary and Slovakia accounted for only 6% of the EU's oil imports from Russia, and the exemptions would not change the impact of the ban on the Russian economy.

Leading EU diplomat Josep Borrell said he would call an extraordinary meeting of EU foreign affairs ministers next week if no deal was reached by the weekend.

Mr Orban said earlier that Hungary would need five years and huge investments in its refineries and pipelines to transform its current system which gets about 65% of its oil from Russia.

One diplomat familiar with the talks among EU envoys in Brussels dismissed Mr Orban's comments as "mostly bluster", describing instead a constructive atmosphere in the negotiations.