Oil prices fell on Wednesday as investor focus returned to worries about oversupply after Kuwaiti workers ended a three-day strike that had halved the nation's crude output.
Industry data showed U.S. crude stockpiles rose last week, reinforcing concerns about the global surplus. Brent futures were down 60 cents at $43.43 a barrel at 1138 GMT.
U.S. crude was down 73 cents at $40.35, after dipping below $40.
Six supertankers have lined up at Kuwait's crude export terminal to load oil, and the country has raised its output to 1.6 million barrels per day (bpd) from 1.1 million on Sunday. The country produced 2.8 million bpd at the end of March.
The end of the strike revived the bearish mood brought on by the failure of major producers on Sunday to agree to freeze output and help overcome a market imbalance that has caused a slump in prices since mid-2014.
"Kuwait is moving back to full production, and we expected that oil would come off more after the Doha deal fell apart, so we're seeing the impact of that now," said Bjarne Schieldrop, chief commodity analyst at SEB in Oslo.
Highlighting a danger that there will be more, rather than less, oil coming from the world's largest exporters, Saudi Arabia and Russia both have potential to steeply increase their output, Russian Energy Minister Alexander Novak said on Wednesday.
Data from the American Petroleum Institute showed U.S. crude stocks rose more than anticipated last week.
Crude inventories increased by 3.1 million barrels in the week to April 15 to 539.5 million, the industry group said, compared with analysts' expectations for a rise of 2.4 million barrels.
Investors will now look to data from the U.S. government's Energy Information Administration for more clues on the outlook for global supplies. The data is due out at 1430 GMT.
(Additional reporting by Aaron Sheldrick in Tokyo and Henning Gloystein in Singapore; editing by Dale Hudson and Jason Neely)