Brent crude was up 20 cents at 77.32 dollars a barrel by 1315 GMT on Monday and United States light crude rose 10 cents to 70.80 dollars.
And reports suggest markets anticipate a sharp fall in Iranian crude supply once U.S. sanctions bite later this year.
But it is unclear how hard the sanctions will hit Iran’s oil industry.
Experts say a lot will depend on how other major oil consumers respond to Washington’s action against Tehran, which will take effect in November.
The surge in oil prices comes at a time of tight supply amid record Asian demand and voluntary output restraint by the Organisation of the Petroleum Exporting Countries and non-OPEC producers, including Russia.