Oil prices tumbled again on Monday, eroding last week's gains, as Opec called for co-operation from oil-producing nations outside the cartel.
Brent crude fell 4.1% to $30.86 a barrel following a 10% rise on Friday, while US oil shed 4.7% to $30.68.
The slide came as the head of Opec called for all oil-producing nations to work together.
Abdullah al-Badri said both Opec and non-Opec oil producers needed to tackle oversupply to help prices rise.
"It is vital the market addresses the issue of the stock overhang. As you can see from previous cycles, once this overhang starts falling then prices start to rise," he told a conference in London.
Despite the ongoing refusal of Saudi Arabia, the dominant Opec member, to cut production, Mr al-Badri nevertheless blamed countries outside the cartel for the huge global oil glut.
"Yes, Opec provided some of the additional supply last year, but the majority of this has come from non-Opec countries," he said.
Opec accounts for almost 42% of the world's oil production.

BBC