LONDON — Under pressure from the British government, the LetterOne Group, an investment vehicle led by the Russian billionaire Mikhail Fridman, said on Sunday that it had sold oil and natural gas assets in the British North Sea to Ineos, a Switzerland-based petrochemical company with no previous experience operating oil and gas fields.

LetterOne acquired the North Sea holdings in March as part of its purchase of DEA, the Hamburg-based unit of the German utility RWE, for 5.1 billion euros, or $5.8 billion. But the British government refused to accept LetterOne as the new owner out of concerns that Mr. Fridman and his partners might in the future be subject to Western sanctions. Britain and other Western countries have imposed a variety of sanctions against Russian interests in response to Russia’s intervention in Ukraine.

Mr. Fridman appears to hope that the sale will allow him to advance his goal of building a global telecommunications and energy conglomerate from a base he has established in London. With Russia under pressure from the West over Ukraine, Mr. Fridman needs a secure international base from which to pursue acquisitions and manage his interests.

There are signs that relations with the government of Prime Minister David Cameron have improved since May, when Mr. Cameron’s Conservative Party won a majority in Parliament and was no longer obliged to govern in a coalition with the Liberal Democrats.