"We aim to break the year's low in crude next week," said Tariq Zahir, an oil bear at Tyche Capital Advisors in Laurel Hollow, New York. Brent fell to $45.19 in January, while US crude dropped to $43.48.

Oil began the day lower after the IEA, which advises industrialized countries on energy, warned the global glut was building and the United States may soon run out of tanks to store crude.

"US supply so far shows precious little sign of slowing down," the IEA said. "Quite to the contrary, it continues to defy expectations."

Some traders also worried about the prospect of Iran reaching a partial nuclear deal with world powers by end March and a full agreement by June. Such a deal could end sanctions against Tehran, enabling it to export more crude, which would suppress prices.

The only real bullish factor on the day was data from oil services firm Baker Hughes showing the number of rigs drilling for oil in the United States fell by 56 this week to 866, the lowest since March 2011. A lower rig count signals a drop in oil output.

News of a US government proposal to buy up to five million barrels of oil for its Strategic Petroleum Reserve also elicited little interest from market bulls.

Brent closed at $54.67, down $2.41, or 4.2 per cent, on the day. US crude finished at $44.84, down $2.21, or 4.7 per cent. Both dropped 9 per cent on the week.

Brent's premium to US crude, a spread that commands one of the biggest volumes in oil, turned volatile, moving from a 10-day high of $11 a barrel to below Thursday's close of $10.