Washington has temporarily allowed the sale of oil from Russia -- one of the world's largest producers -- that is at sea, as it tries to contain oil prices pushed up by the Iran war.

But that has prompted backlash from Western allies who say the proceeds could be used to fund Russia's invasion of Ukraine.

"The unilateral decision by the US to lift sanctions is very concerning as it impacts European security," Costa wrote on X.

"Increasing economic pressure on Russia is decisive for it to accept a serious negotiation for a just and lasting peace," added Costa, who presides over the European Council representing the 27 member states.

"Weakening sanctions increases Russian resources to wage the war of aggression against Ukraine," he added.

US-Israel strikes on Iran and Tehran's retaliatory attacks across the Gulf region have virtually halted transit through the strategically vital Strait of Hormuz, sending global oil prices spiralling.

Russia has benefitted, with the high prices helping Moscow to cover a widening gap in public finances.

European Commission spokeswoman Paula Pinho told a press conference Friday Moscow has been pocketing $150 million a day in extra revenues from oil sales since the beginning of the Iran war.

"It is not the time to relax sanctions on Russia," she said, describing the Kremlin as "probably the biggest beneficiary" of the conflict.

The European Union banned maritime imports of Russian crude in 2022, while pipeline exports via Ukraine to Hungary and Slovakia have been effectively blocked since January.