Brent crude tumbled below $50 today, heading for a second week of losses in a row on worries that Donald Trump's decision to abandon a climate pact could spark more crude drilling in the US, worsening a global glut.
Benchmark Brent crude futures were off by nearly 3% at $49.14 per barrel, down $1.49 from the previous close.
US West Texas Intermediate crude futures fell $1.45 cents to $46.91 per barrel.
Both contracts were on track for weekly losses of more than 5%.
The US withdrawal from the landmark 2015 global agreement to fight climate change drew condemnation from Washington's allies - and sparked fears that US oil production could expand even more rapidly.
US crude production last week was up by nearly 500,000 barrels per day (bpd) from year-earlier levels, straining OPEC's efforts to reduce global oversupply.
A week ago, the Organisation of the Petroleum Exporting Countries and a number of non-OPEC producers met in Vienna to extend a deal to cut 1.8 million bpd from the market until March 2018.
Igor Sechin, chief of Russia's largest oil producer, Rosneft, said today that US oil producers could add up to 1.5 million bpd to world oil output next year.
Oil prices are down some 10% since OPEC's May 25 decision to extend the cuts.
Rising output from OPEC members Nigeria and Libya, which are exempt from the output reduction deal, is also undercutting attempts to limit production.
OPEC last week discussed reducing output by a further 1-1.5%, and could revisit the proposal should inventories remain high, sources told Reuters.
However, oil markets received some support from official US data which showed crude inventories fell sharply last week as refining and exports surged to record highs.
Crude stockpiles were down by 6.4 million barrels in the week to May 26, compared with analysts' expectations for a fall of 2.5 million barrels.