U.S. President Donald Trump spoke with Saudi Arabian Crown Prince Mohammed Bin Salman by phone Monday to discuss the oil-price war, according to two people familiar with the call.

The discussion comes as Saudi Arabia and Russia engage in a battle of economic might over the global oil market, and endanger the U.S. shale industry as a result.

The White House later confirmed the call.

The 34-year-old Saudi crown prince escalated a price war Tuesday with a move to flood the market with crude, a day after the oil market suffered its deepest rout since 1991.

Russian President Vladimir Putin helped kick off the fight in the first place to inflict a blow on the U.S.