OPEC, which has reduced output even more than it had pledged last year, also said in its monthly report that US shale output and other non-OPEC supply was increasing.
Brent futures were down 67 cents at $50.68 a barrel by 1321 GMT (6:51 pm in India), after earlier touching $50.55, their lowest since November 30.
US crude slid 82 cents to $47.58 a barrel after dropping during the session to $47.47, also the lowest since November.
Oil prices have now reversed almost all the gains notched up since OPEC said on November 30 that it was cutting output.
Despite OPEC sticking to its pledge to cut, inventories have continued to rise. Weekly data due to be released on Tuesday by the American Petroleum Institute (API) is expected to show another rise in US inventories, following on from last week's report that showed a bigger than expected increase.
"Right now, the direction of travel is more important than the absolute level," said Harry Tchilinguirian, global head of commodity strategy at BNP Paribas, citing concerns that OPEC-led cuts lasting from January to June had not halted the accumulation of inventory.
OPEC has said its supply cuts aim to stabilise prices and draw down global inventories. Investors are now looking for indications that OPEC will extend its reductions beyond June.
Saudi ally Kuwait said on Monday it would support an extension of the global deal.
"It will speed up rebalancing of the market and will help bring prices to acceptable levels for oil-producing nations and the industry in general," the Gulf state's oil minister, Essam al-Marzouq, told the state news agency.
But Saudi Arabia, the world's biggest oil exporter, which has cut even more production than it had pledged in the OPEC deal, has yet to indicate clearly whether it is ready to extend.