In earlier deals, US benchmark West Texas Intermediate (WTI) for April delivery dived as low as $43.
57 — hitting the lowest since March 12, 2009.
The contract later recovered slightly to stand at $44.
40, down 44 cents from Friday's closing level.
European benchmark Brent North Sea crude for April fell 79 cents to $53.
38 a barrel in midday London trade.
Singapore-based Phillip Futures said "with fundamentals remaining unchanged, we hardly find a reason for crude oil to break higher or even lower".
Crude prices lost some 60 percent of their value to decline to about $40 per barrel between June and late January owing to an oversupply in world markets, a weak global economy and the soaring dollar. Prices have since rebounded following a slowdown in US oil-drilling activities, but analysts say volatility is likely to continue for some time.
A stronger US dollar makes dollar-priced oil more expensive for buyers using weaker currencies, denting demand.