Energy markets are undergoing an extended sell-off, the longest this year, with new economic reports dampening optimism about any economic recovery.

Benchmark crude for August delivery fell 98 cents to US$61.95 on the New York Mercantile Exchange mid-morning.

In just over one week, oil prices have fallen more than 15 per cent.

And on Wednesday, the Or-ganiSation of Petroleum Exporting Countries predicted that demand for crude has fallen so sharply, it will take another four years to recover to 2008 levels.

"So far there is no clear evidence of oil demand returning," said Victor Shum, an energy analyst with Purvin & Gertz.

This year, that hasn't seemed to matter, with billions flowing into energy markets because of the weak dollar. Investors have used crude as a hedge against inflation, but that has driven energy prices higher.

-AP