LONDON (Reuters) - OPEC and its allies hold talks on Thursday to discuss plans for the biggest coordinated oil output cut in history but two hurdles remain: reaching a deal on the levels from which to make reductions and securing the participation of the United States.

FILE PHOTO: The logo of the Organisation of the Petroleum Exporting Countries (OPEC) is seen at OPEC's headquarters in Vienna, Austria July 1, 2019. REUTERS/Leonhard Foeger

U.S. President Donald Trump said last week he had brokered a deal between Russia and Saudi Arabia that could see cuts of 10 million to 15 million barrels per day (bpd), or 10 to 15% of global supplies. But Washington has not said it will join in.

The coronavirus crisis, meanwhile, has slashed oil demand by 30%.

ALL ABOUT THE BASELINE

OPEC sources have told Reuters a key element to the discussions is reaching a deal on what levels of national production to use to calculate any output cuts.

An OPEC source said discussion were around whether to cut from April levels or whether to use a period before that.

Another source said Riyadh, which hiked supply to a record 12.3 million bpd in April from 9.8 million bpd in March, was insisting on cutting production from April levels.

Russian President Vladimir Putin has said he favoured cuts being made from production levels in the first quarter.

“Clearly, cuts taking place from the most recent elevated levels would have less impact on absolute supply,” FGE Energy said.

In terms of volumes, two Russian sources said the maximum Russian cut would be 2 million bpd or about 17% of output. Saudi Arabia has yet to indicate how much it is prepared to cut.

THE PRODUCTION DATA CHALLENGE

One of the challenges facing OPEC+ - the informal grouping of the Organization Petroleum Exporting Countries, Russia and other producers - is getting reliable production figures for producers.

OPEC’s monthly oil market report on output from its members relies on secondary sources including price reporting agencies, the International Energy Agency (IEA) and the U.S. Energy Information Administration (EIA) to assess production.

Secondary source data is sometimes at odds with the direct submissions of individual members, which has allowed some members like Nigeria and Iraq to justify lagging behind on compliance with previous cut agreements.

The data usually carries a lag of up to two weeks, so February’s production figures were published on March 11 and the March figures are expected to come out on April 16.

Another challenge for OPEC+ has been around how some members assess their production. After much debate, Russia secured agreement among OPEC+ to include oil and condensate - a by product in crude production - in its total output level.

THE U.S. CONUNDRUM

Russia has insisted that it would only accept cuts to its output if the United States joined in with reductions.

The U.S. Department of Energy said U.S. output was already falling without government intervention, in line with the position of the White House that it would not intervene.

U.S. Department of Energy projections now show U.S. oil output averaging 11 million bpd in 2021, which correlates to about a 2 million bpd decline from the late 2019 peak.

But Moscow has said natural production declines were not the same as the voluntary output cuts now being negotiated.

Adding to the political mix, U.S. Republican lawmakers piled pressure on Riyadh on Wednesday to cut production. Nearly 50 Republican lawmakers told Saudi Crown Prince Mohammed bin Salman that economic and military cooperation between the two countries was at risk unless the kingdom helped stabilise oil prices.