OPEC has urged its members not to mention oil prices when discussing policy. The oil producing group is seeking to avoid the risk of U.S. legal claims it manipulates the market, sources close to OPEC said.

Proposed U.S. legislation known as "NOPEC," which could open the group up to anti-trust lawsuits, has long lain dormant, with previous American presidents signaling that they would veto any move to make it law.

But President Donald Trump has been a vocal critic of the Organisation of the Petroleum Exporting Countries, blaming it for high oil prices and urging it to increase output to relieve pressure on a market hovering around four-year highs.

That has made OPEC and its unofficial leader, Saudi Arabia, nervous about what it might mean for NOPEC, or No Oil Producing and Exporting Cartels Act.

The decision to refrain from discussing a preferred oil price level — one way the group can guide market expectations — underlines how Trump's aggressive stance on the oil market is unsettling OPEC and testing ties between allies Riyadh and Washington.

In July, senior OPEC officials attended a workshop in Vienna with international law firm White & Case to discuss the NOPEC bill, and the lawyers advised avoiding public discussion of oil prices and rather talk about the stability of the oil market, two sources familiar with the matter said.

OPEC officials were also advised to explore diplomatic lobbying channels to try and prevent the NOPEC bill from becoming law, one of the sources said.

On Aug. 1, the OPEC secretariat sent a letter to the ministers making a similar recommendation.

"We solemnly believe that market stability, and not prices, is the common objective of our actions," UAE Energy Minister Suhail al-Mazroui, who holds the rotating OPEC presidency this year, wrote in the letter, seen by Reuters.

"I would like to call upon OPEC Member Countries, as well as our participating Non-OPEC colleagues, to refrain from any reference to prices in their commentary about our collective efforts or oil market condition," he added.

White & Case did not respond to a Reuters request for comment.

Specifying oil prices is not the only way OPEC tries to guide the market. By cutting production it can support prices and by raising supplies it can do the opposite, for example.

But the private coordination of how to communicate OPEC's message to the market represents a departure from past practice, when Saudi Arabia would often signal a preferred price level when speaking about OPEC policy and seek to push through actions to achieve that.

While chances of the law passing this year appear slim, concerns among OPEC members and other oil producers are growing that it may ultimately get the support of Trump, given his open criticism of OPEC and high oil prices.

The OPEC letter came two months before U.S.-Saudi relations were further strained when a Saudi journalist disappeared during a visit to the kingdom's consulate in Istanbul.

With close to $1 trillion in investments in the United States, including assets owned by Saudi Aramco, Riyadh has a lot to lose if the NOPEC bill were passed into law.

It would revoke the sovereign immunity which oil producers, including OPEC members, currently enjoy from U.S. legal action.