Oil collapsed into free-fall ovrnight, diving more than 10 per cent and sending US crude back under $US100 a barrel as investors staged an unprecedented stampede for the exits.

US crude settled down $US9.44 at $US99.80 a barrel, dropping down to $US98.26 a barrel in post-settlement trade, marking the second-biggest one day loss in dollar terms on record.

Weak economic data from Europe and the United States fed concerns that have battered commodities all week. German industrial orders fell unexpectedly in March while US weekly jobless claims hit eight-month highs.

But the onslaught of selling went far beyond any single cause. Brent crude plunged more than $US12 at one point - exceeding the sell-off that followed Lehman Brothers' collapse.

US crude broke below $US100 for the first time since March as technical triggers set off a cascade of sell-stops.