Oil prices rose on Friday ahead of a meeting of OPEC and other large crude exporters that will focus on production increases as U.S. sanctions restrict Iranian exports.
OPEC and its allies are scheduled to gather in Algeria on Sunday to discuss how to allocate higher supply to offset the shortage of Iranian supplies.
Benchmark Brent crude oil was up 65 cents at $79.35 per barrel by 0855 GMT. U.S. light crude was up 45 cents higher at $70.77.
Brent is close to four-year highs, trading just below $80 a barrel, as investors bet that the Organization of the Petroleum Exporting Countries will be unable to compensate fully for the loss of oil from Iran, OPEC's third biggest producer.
But the meeting on Sunday is unlikely to be able to change production policy. Such a move would require OPEC to hold what it calls an "extraordinary meeting," which is not on the agenda.
U.S. President Donald Trump increased pressure on OPEC on Thursday, calling on OPEC to "get prices down now!"
"We protect the countries of the Middle East, they would not be safe for very long without us, and yet they continue to push for higher and higher oil prices," Trump tweeted.
Trump's remarks ahead of the OPEC meeting put "a focus on the likely supply impacts of U.S.-led Iran sanctions," said Stephen Innes, head of trading for Asia-Pacific at OANDA.
"The market had until that point been trading fluidly with the assumption that Saudi Arabia is now comfortable with Brent at $80 or even higher, which is challenging the market's long-held supposition that prompt Brent between $70 and $80 was OPEC's sweet spot," Innes said.
Tanker tracking and industry data show Iranian exports of crude have already begun to decline well before the imposition of new U.S. sanctions on Tehran.
"Iranian crude exports are coming (down) earlier and bigger than expected, at a time seasonal demand is strong. With spare capacity also falling sharply, the market remains exposed to supply-induced price shocks," ANZ Bank analysts said in a note to clients.
Jefferies bank analyst Jason Gammel said he expected Saudi Arabia, OPEC's biggest producer, to try to keep the oil market adequately supplied into 2019, "but at the cost of spare capacity," a key supply buffer to prevent oil price volatility.
"Based on the definition of achievable within 20 days and sustainable for 90 days, spare capacity could fall below 1 percent of demand by year-end if Iranian exports fall below 1 million barrels per day, as now seems likely," Gammel said.
(Reporting by Christopher Johnson in London, Jane Chung in Seoul and Aaron Sheldrick in Tokyo; editing by Jason Neely)