The price of oil rose above $100 a barrel on Monday amid renewed tensions over Ukraine.
Benchmark U.S. crude for June delivery gained 60 cents to $100.59 a barrel on the New York Mercantile Exchange.
Brent crude, a benchmark for international varieties, rose 52 cents to $108.41 on the ICE Futures exchange in London.
Traders are worried about turmoil in Ukraine and the prolonged disruption of oil exports from Libya despite muted demand and plentiful supplies.
Pro-Moscow insurgents in eastern Ukraine declared independence Monday and sought to join Russia. The Ukrainian and Western governments have condemned the rushed referendum as a sham and a violation of international law.
There are concerns that Russian energy exports to Europe could be interrupted if further instability in Ukraine results in stronger Western sanctions. On Monday, Moscow said it had no intention to annex Ukraine's eastern provinces, while the European Union added 13 people to its list of 48 Russians and Ukrainians punished with visa bans and asset freezes over the Ukraine crisis.
In other energy futures trading in New York:
— Wholesale gasoline added 2 cents to $2.91 a gallon.
— Heating oil gained 1 cent to $2.92 a gallon.
— Natural gas fell 10 cents to $4.43 per 1,000 cubic feet.