Signs of weakness in the global economy are also upsetting investors. China's factory output shrank again in December, a preliminary purchasing managers' survey showed, reinforcing concerns that manufacturers face waning global demand and tight domestic credit.
Supply concerns arising over Iran as tension in the Middle East over Tehran's nuclear programme grows remained a supportive factor.
In a sign the Islamic Republic may struggle to redirect European volumes to Asia in the event of sanctions, Turkey's Halkbank refused to open an account for India's BPCL to settle payments for oil imports from Iran.
OPEC oil producers on Wednesday agreed to an output target of 30 million barrels per day, ratifying current production near 3-year highs, in a deal that settles a 6-month-old argument over supply policy firmly in Saudi Arabia's favour.
But OPEC did not discuss individual nations' quotas, and there remains no mechanism in place to cut quotas should already-fragile demand grow less quickly than expected.
(Additional reporting by Gene Ramos and Manash Goswami; editing by Keiron Henderson)