During a meeting in Moscow last month, Russian President Vladimir Putin and Venezuelan President Nicolas Maduro, who has already threatened to nationalize Chevron's assets, discussed the possibility of Rosneft taking over Chevron's Venezuelan operations, sources said.

Though trade between Venezuela and Cuba has not ceased amid sanctions, the flow of PDVSA’s oil has been insufficient to keep the Communist-ruled island well-supplied. Cubans have faced long lines at gas stations and for public transportation amid lack of vessels to receive imports.

Rosneft, which produces around five percent of the world’s oil, is now taking care of shipping and marketing operations for the bulk of Venezuelan oil exports, ensuring that PDVSA can continue to supply buyers.

The United States has targeted Cuba’s state-run company Cubametales as part of its Venezuela-related sanctions, according to a notice posted on the U.S. Treasury Department’s website on Wednesday.

PDVSA and its joint ventures exported 1.1 million barrels per day (bpd) of crude and refined products last month, a 26% increase over May. Chinese buyers took 59% of the shipments, followed by India with 18% and Singapore with 10%, the documents showed.

Venezuela's exports of crude oil and refined products fell 17% in May from the previous month to 874,500 barrels per day

Venezuela's state-owned PDVSA exported 1.416 million barrels of combined crude and products in May to Cuba's state Cubametales, up from 355,000 barrels in April.

Crudes are being priced at Dated Brent discounts between $15/b and $17/b, FOB, according to a PDVSA report.

Despite significant U.S. oil sanctions on the OPEC member nation, Maduro has held on to power, backed by Cuba, Russia and China, and retains the support of the country’s military and other institutions.

OPEC is due to meet in June, where the Iran question will no doubt be raised. But oil and gas leaders are taking a cautious approach, with Kuwait's oil minister Khaled Al Fadhel noting that a decision would be taken only after a review of oil prices.