* "OPEC monopoly must get prices down now!," Trump tweets
* OPEC gathering unlikely to agree change in oil output
* U.S. sanctions on Iran oil exports start on Nov. 4 (Adds comment, updates prices, changes dateline to NEW YORK, previously LONDON)
NEW YORK, Sept 20 (Reuters) - Oil prices eased on Thursday, slowing an upward surge that had pushed the market toward four-year highs, after U.S. President Donald Trump called on OPEC to "get prices down now!"
Brent crude oil was down 50 cents at $78.90 a barrel by 11:32 a.m. EDT (1532 GMT). U.S. light crude was down 9 cents at $71.03 a barrel after rising nearly 2 percent on Wednesday.
Global benchmark Brent has been trading just below $80 a barrel, near its highest in almost four years, on expectations that U.S. sanctions against Iran, OPEC's third biggest producer, will reduce global supply.
"We're stuck in a range here," said Tariq Zahir, managing member at Tyche Capital Advisors in New York, with prices supported by sanctions on Iranian supplies and pressured by the potential Chinese demand could wane. U.S. sanctions were imposed by Trump in response to Iran's nuclear programme. 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 to an official rise in crude output, although pressure is mounting to prevent a spike in prices.
Trump weighed into the debate via Twitter, saying "The OPEC monopoly must 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! We will remember."
In the range-bound market, Trump's tweets may drive prices down slightly, said Tyche's Zahir, predicting that such tweets would probably continue with U.S. midterm elections approaching.
"The last thing that he is going to want to have happen is diesel prices and oil prices hitting highs when people are going to the voting booth," Zahir said.
Still, many traders and analysts expect 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."
U.S. sanctions on Iran's oil exports come into force on Nov. 4. Many buyers have already scaled back Iranian purchases, and it was unclear how easily other producers, such as Saudi Arabia, Iraq and Russia, can compensate for lost supply.
BNP Paribas oil strategist Harry Tchilinguirian told 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."
(Reporting by Christopher Johnson in London, Aaron Sheldrick and Osamu Tsukimori in Tokyo Editing by David Gregorio and David Evans)