At about 1600 GMT, US benchmark West Texas Intermediate (WTI) for delivery in May was down 23 cents at USD 36.56 a barrel.
Brent North Sea crude for June delivery shed 41 cents to USD 38.26 a barrel compared with today's close.
Hopes for a deal at the April 17 gathering in Doha led by Russia and Saudi Arabia had been a major driver of a rally in prices from near 13-year lows in February.
"If all countries agree to freeze production, we're ready," he said in an interview with Bloomberg News.
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Michael McCarthy, an analyst with CMC Markets in Sydney, told AFP the price slump came as "no surprise" as traders reacted to Salman's comments.
"Part of the recent strong rally has been related to some optimism in the agreement and that's now looking less likely following those comments from the Saudis," he said by telephone.
Iran has been ramping up production after nuclear-linked Western economic sanctions were lifted in January, adding to the saturated market. Iranian officials have made clear they plan to ramp up production to pre-sanctions levels.
"There were renewed concerns over continuing oversupply due to comments from Saudi Arabia that it will agree to freeze production only if Iran and other major producers agree to do the same," said EY oil analyst Sanjeev Gupta.