By Christopher Johnson
Benchmark Brent crude jumped $1.49 to a high of $79.34 a barrel before easing back to around $78.75 by 1230 GMT.
U.S. light crude was unchanged at $73.45. On Thursday, the U.S. oil futures contract hit its highest since November 2014 at $74.03 per barrel.
Iran is the fifth-largest oil producer in the world, pumping about 4.7 million barrels per day (bpd), or almost 5 percent of total output, much of it to China and other energy-hungry nations such as India.
The U.S. government wants to stop Tehran exporting oil to cut off a vital supply of finance, and hopes other big oil producers in the Organization of the Petroleum Exporting Countries and Russia will make up for the deficit.
But the world oil market is already tight with unplanned disruptions in Canada, Libya and Venezuela removing supply.
Many analysts and investors think strict enforcement of U.S. sanctions against Iran will push up prices sharply.
"It is becoming increasingly clear that Saudi Arabia and Russia will struggle to compensate for potential losses in oil production from the likes of Venezuela, Iran and Libya," said Abhishek Kumar, analyst at Interfax Energy in London.
Vienna-based consultancy JBC Energy said the stronger the implementation and enforcement of U.S. sanctions, the higher the oil price will go:
"Triple-digit oil prices are not off the table," JBC said.
A Reuters survey of 35 economists and analysts on Friday forecast Brent would average $72.58 a barrel in 2018, 90 cents higher than the $71.68 forecast in last month's poll and compared with the $71.15 average so far this year.
North American oil stocks have fallen as an outage at Canada's Syncrude has locked in more than 300,000 bpd of production. The outage is expected to last at least through July, according to operator Suncor Energy.
Outside North America, record demand and voluntary supply cuts led by OPEC have pushed up prices.
Major buyers of Iranian oil, including Japan, India and South Korea, have indicated they may stop importing Iranian crude if U.S. sanctions are imposed.
Until then, however, they are buying as much Iranian oil as possible. Imports of Iranian crude oil by major buyers in Asia rose in May to the highest in eight months.
(Additional reporting by Henning Gloystein in Singapore; editing by Louise Heavens)
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)