DUBAI/LONDON (Reuters) - The world's top oil producers Saudi Arabia, Russia and the United States still seemed at odds on Wednesday before this week's meetings on potentially big output cuts to shore up crude prices that have been hammered by the coronavirus crisis.

Saudi Arabia and Russia, which fell out when a previous pact on curbing supplies collapsed in March, have signaled they could agree deep cuts to crude output but only if the United States and others outside a group known as OPEC+ joined in.

But the U.S. Department of Energy said on Tuesday that U.S. output was already falling without government action, echoing views from the White House that it would not intervene, even as global demand for crude has plunged by as much as 30%.

U.S. President Donald Trump said last week a deal he had brokered with Saudi Arabia and Russia could lead to cuts of as much as 15% of global supplies, an unprecedented level.

Yet Moscow and Riyadh have yet to publicly indicate any agreement on the level of any reductions or how to distribute them among OPEC+, which includes the Organization of the Petroleum Exporting Countries, Russia and other producers.

"Let's wait for tomorrow or the day after," said Kremlin spokesman Dmitry Peskov, when asked about Russia's position before Thursday's OPEC+ talks.

Energy ministers from the Group of 20 nations (G20) also hold a video conference.

Moscow, Riyadh and others need to agree on what national production level, or baseline, for calculating cuts, an issue muddied since last month's acrimonious OPEC+ meeting in Vienna that led to limits being scrapped and race for market share that flooded an already oversupplied market with extra crude.

Saudi Arabia ramped up output to a record 12.3. million bpd in April, up from below 10 million bpd in March. But Russia has said any cut should be calculated based on first-quarter levels.

"The issue is still the baseline," an OPEC source said.

Asked about what baselines were being negotiated, the source said: "April versus anything else before April."

Russian TASS news agency said any cuts could last three months starting from May. Others have not indicated how long they could run.

AGGRAVATING THE MARKET

Iran, which was exempted from the previous OPEC+ deal, said details such as the baseline and the contributions by the United States and others Canada, should have been agreed before any meeting was scheduled.

Iranian Oil Minister Bijan Zanganeh said that, "in the absence of any clear and consensual outcome", a failure of talks could "aggravate the current low price environment even further".

Oil prices, which fell to their lowest in almost two decades in March, are still trading at half their level at the end of 2019, before the coronavirus crisis prompted governments to tell people to stay home and fuel demand plummeted.

The United States, whose production has surged in recent years with the help of the revolution in shale oil production and helped by past OPEC+ efforts to support prices, has said the decline in its output would take place slowly, over two years.

U.S. Department of Energy projections 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.

U.S. antitrust law stops producers in the United States from reining in output to prop up prices, but state regulators or the federal government can order lower production levels.

U.S. producer Occidental Petroleum said it opposed any production limits introduced by regulators in Texas, the largest-producing U.S. state. Regulators there meet next week to consider production cuts.

Reflecting U.S. political pressures, Republican senators introduced a bill to remove U.S. defense systems and troops in Saudi Arabia unless the kingdom cut output. The senators were due to speak to Saudi officials on Saturday, a source said.

Any final agreement for how much OPEC+ will cut during Thursday's talks would depend on volumes other producers such as the United States, Canada and Brazil offered to cut, OPEC sources said.

Some producers outside OPEC+ have indicated a willingness to help. Energy firms in Canada, home to the world's third-largest oil reserves, have already been reducing output due to the sharp fall in prices.