LONDON (Bloomberg) --Saudi Arabia, Russia and other large oil producers are racing to negotiate a deal to stem the historic price crash as diplomats said some progress was made on Sunday.

The talks still face significant obstacles: a meeting of producers from OPEC+ and beyond -- delayed once -- is only tentatively scheduled for Thursday. Russia and Saudi Arabia want the U.S. to join in, but U.S. President Donald Trump has so far shown little willingness to do so.

Oil diplomats are trying to stitch together a meeting of G-20 energy ministers for Friday, part of an effort to bring the U.S. on board, according to two people familiar with the situation. Even the International Energy Agency, which represents industrialized energy-consuming nations, is calling for action.

“We see a huge oversupply in the oil market,” Fatih Birol, the head of the IEA, said in an interview on Sunday. “There’s a need for the G-20 in the driving seat, led by its current chair, Saudi Arabia.”

Crude prices have fallen 50% this year, as the economic effects of the coronavirus pandemic have knocked out about a third of global demand. The price crash is so dramatic that it’s threatening the stability of oil-dependent nations, the existence of U.S. shale producers, and poses an extra challenge to central banks. Industry officials say that if a deal to cut supply in an orderly way isn’t reached, the market will simply force producers to slash output as storage space runs out.

The aim of talks, first revealed by Trump last week, is to cut oil production by about 10% -- the biggest ever coordinated reduction. Crude rallied on Trump’s comments but pared those gains as the diplomatic intricacies became clearer. Brent futures fell 1.2% on Monday to $33.74 a barrel.

Russia and Saudi Arabia are “very, very close” to reaching a deal on oil-production cuts, Kirill Dmitriev, chief executive officer of the Russian Direct Investment Fund, said in an interview with CNBC.

However, even if a deal is struck for as much as 10 million barrels per day, that will barely dent the supply glut, which is estimated at as much as 35 million barrels a day. In some corners of the physical market prices have already turned negative, and traders have been putting oil into tankers at a record pace to store it at sea.

Saudi Arabia and Russia both say they want the U.S., which has become the world’s largest producer thanks to its shale revolution, to join the cuts. But Trump had only hostile words for OPEC on Saturday, threatening tariffs on foreign oil, though at a briefing late Sunday he said he didn’t expect he’d have to use them. G-20 may be an easier forum for the U.S. to embrace than OPEC.

“If the Americans don’t take part, the problem which existed before for the Russians and Saudis will remain -- that they cut output while the U.S ramps it up, and that makes the whole thing impossible,” said Fyodor Lukyanov, head of the Council on Foreign and Defense Policy, a research group that advises the Kremlin.

It’s not clear if Russia and Saudi Arabia will require the U.S. to publicly commit to cut production -- a challenge in the private, fragmented American industry -- or if a compromise gesture would be enough. Alexander Dynkin, president of the Institute of World Economy and International Relations in Moscow, a state-run think tank, said Moscow would like the U.S. to lift some sanctions as a compromise.

Even a passive role for the American shale industry, whose output is already expected to go into decline at current prices, may be enough for a deal, said Jason Bordoff, director of the Center on Global Energy Policy at Columbia University and a former oil official at the White House.

”Russia and Saudi Arabia’s condition that they will only cut production if the U.S. does too is going to be satisfied, because market forces will drive U.S. output down around 1 million barrels a day this year,” said Bordoff.

Back Channels. Russia and Saudi Arabia -- which sparred publicly between themselves over the weekend -- have disagreed about how they would calculate the cuts, according to a person familiar with the talks.

Russia favors using an average of the first quarter output as the baseline, while Saudi Arabia wants to use its current April production. The difference is huge: the kingdom pumped 9.8 million barrels a day on average between January and March. In April -- as it wages its battle for market share -- it’s producing more than 12 million.

Any agreement will require diplomatic agility at a time when nations are devoting massive resources to fighting the pandemic itself. All three players -- Crown Prince Mohammed bin Salman, Russian President Vladimir Putin and Trump -- appeared to be maneuvering to avoid blame if talks fail. Yet the U.S. president has also said he’s confident there’ll be an agreement between Moscow and Riyadh to cut production.

“The chances of a meaningful deal that delivers real production cuts are low but back-channel talks are ongoing,” said Amrita Sen, chief oil analyst at consultant Energy Aspects Ltd. “Mohammed bin Salman is under heavy political pressure from Trump to demonstrate the Kingdom isn’t trying to bankrupt the U.S. shale industry.”