Oil prices have fallen below $49 a barrel amid signs of supply glut outweighing hopes that oil producing countries will agree a proposal to support prices.
Brent crude fell 59 cents at $48.57 a barrel, while the US crude dropped 62 cents at $46.79, Reuters reported.
A militant group in Nigeria claiming responsibility for a series of attacks on oil facilities and pipelines in the oil-rich Niger Delta has since said that it was ready for a ceasefire.
Furthermore, prices also fell due to Iraq restarting pumping oil through a northern pipeline that was halted earlier this year.
The news agency quoted Goldman Sachs saying that discussions of an output freeze by the Organization of the Petroleum Exporting Countries (OPEC) and a weak dollar helped prices increase this month.
Due to excess supply globally, oil prices have more than halved since mid-2014.
In April, a meeting by major oil producers to freeze output failed due to tensions between Iran and Saudi Arabia, as well as the refusal of some countries to participate.
Investors are watching for data to be released by the American Petroleum Institute and are also seeking clues on whether the US Federal Reserve will increase interest rates this year.