Brent North Sea crude for October sank as low as USD 101.16 per barrel, reaching a level last seen on June 26, 2013. It later stood at USD 101.23, down USD 2.30 from Friday's closing level.
Also today, US benchmark West Texas Intermediate (WTI) for delivery in September tumbled USD 1.39 to USD 95.96 a barrel.
"Crude oil was having another day of heavy losses after a slight respite from recent declines on Friday," said CMC Markets analyst Jasper Lawler.
The oil market had jumped late on Friday after Kiev said that Ukrainian forces had destroyed part of a Russian military convoy that entered its territory.
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"It became apparent on Friday just how nervously the market reacts to geopolitical risks: reports of the alleged destruction of an armed Russian convoy by Ukrainian troops on Ukrainian territory pushed Brent up by one US dollar," said Commerzbank analyst Carsten Fritsch.
News of the shelling had sent stock markets tumbling and pushed up oil prices on Friday as investors feared direct conflict could break out between Ukraine and Russia, the world's second-largest crude producer.
But European equities rebounded sharply on Monday as traders breathed a sigh of relief over easing tensions in east Ukraine and gains by Western-backed forces in Iraq.
Russia said today that "certain progress" was made at Ukraine crisis talks in Berlin between the two countries' foreign ministers and their counterparts from Germany and France.
Elsewhere, traders eyed easing market concerns over ongoing unrest in key OPEC crude producer Iraq.