The price of crude for November delivery on the New York Mercantile Exchange hit $55.33 a barrel - a gain of 70% since January.
Analysts have said they are now eyeing a price of $60 a barrel by the end of the year.
Federal Reserve head Alan Greenspan has said high oil prices do not yet pose a serious threat to the US economy.
'Unsustainable'
He noted last week that after the 1979 Iranian revolution, oil leapt to $80 a barrel in today's money.
But, he warned, further rises could pose a significant risk, in terms of both growth and inflation.
On Monday in Australia, analyst at ANZ Bank in Melbourne, Daniel Hayes, said the current oil prices were "unsustainable".
"There is no doubt that at some point in the next six months we are going to see a price retreat," he said.
But Victor Shum, oil analyst at the US-based energy consultants Purvin & Gertz said $60 a barrel is "certainly feasible".
"We've hit the new milestone."
Bank of Japan Governor Toshihiko Fukui said on Monday that he expected Japan's economy to continue to recover but that "close attention" must be paid to the rising oil prices.
A Japanese government report last week said economic recovery could be affected by the high price of oil, as well as by possible slowdowns in demand from China and the US.