The U.S. sanctions were imposed by Trump in response to Iran's nuclear programme, which the White House says is designed to produce weapons, an allegation Tehran denies.
The Organization of the Petroleum Exporting Countries and other producers, including Russia, meet on Sunday in Algeria to discuss how to allocate supply increases to offset the loss of Iranian barrels.
The meeting is unlikely to agree an official rise in crude output, although pressure is mounting on top producers to prevent a spike in prices.
Trump weighed into the debate via Twitter on Thursday, urging OPEC to cut prices.
"The OPEC monopoly must get prices down now!," Trump said.
"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! We will remember."
Despite the president's intervention, market sentiment remained bullish with many traders and analysts expecting Brent to move above $80 soon.
"Brent is definitely fighting the $80 line, wanting to break above," said SEB Markets chief commodities analyst Bjarne Schieldrop. "But this is likely going to break very soon."
BNP Paribas oil strategist Harry Tchilinguirian agreed, telling Reuters Global Oil Forum: "$80-per-barrel Brent is a psychological level, and unsurprisingly, as we approach it, it gets sold into as some market participants take profit."
"As more evidence gathers that Iranian oil exports are heading sharply down, the more emboldened the oil market is likely to be to test, and breach this level."
U.S. crude oil stockpiles fell for a fifth straight week to 3-1/2 year lows in the week to Sept. 14, while gasoline inventories also showed a larger than expected draw on unseasonably strong demand, the Energy Information Administration said on Wednesday.
Crude inventories fell by 2.1 million barrels, compared with expectations for a decrease of 2.7 million barrels.
U.S. sanctions on Iran's oil exports come into force on Nov. 4 and many buyers have already scaled back Iranian purchases.
It is unclear how easily other producers, such as Saudi Arabia, Iraq and Russia, can compensate for lost supply.
(Reporting by Christopher Johnson in London, Aaron Sheldrick and Osamu Tsukimori in Tokyo Editing by David Evans and Edmund Blair) ((email@example.com; +44 7790 561 651; Reuters Messaging: firstname.lastname@example.org))