* Washington's Iran oil export sanctions to start on Nov. 4
* OPEC seen having little spare capacity to fill Iran loss
* Canada-U.S. NAFTA deal lifts market sentiment
* HSBC says high oil price to weigh on growth
* Coming Up: API's weekly oil inventory data at 4:30 p.m./2030 GMT (Updates with settlement prices)
By Jessica Resnick-Ault
NEW YORK, Oct 2 (Reuters) - Oil prices eased slightly on Tuesday after rallying for three straight sessions, but remained close to four-year highs on worries that global supplies will drop due to Washington's sanctions on Iran.
"This is the market catching its breath," said Gene McGillian, director of market research at Tradition Energy in Stamford, Connecticut.
In addition to the worries that Iran, prices are being supported by global demand that has remained strong in the face of trade tensions.
Brent fell 18 cents to settle at $84.80 per barrel, a day after hitting a four-year high of $85.45. U.S. West Texas Intermediate (WTI) crude futures were off 7 cents at $75.23 a barrel, after earlier touching a four-year high of $75.91.
Analysts polled by Reuters forecast that U.S. crude stocks rose about 2 million barrels last week ahead of data from industry group the American Petroleum Institute (API) due out at 4:30 p.m. and from the U.S. government on Wednesday morning.
Crude prices have roughly tripled from lows hit in January 2016 after the Organization of the Petroleum Exporting Countries and allies led by Russia cut output.
Oil market sentiment was lifted by Sunday's last-gasp deal to salvage NAFTA as a trilateral pact between the United States, Mexico and Canada.
The U.S. sanctions against Iran's oil industry, which at its peak this year supplied nearly 3 percent of the world's daily consumption, are due to go into effect on Nov. 4.
A Reuters survey of OPEC production found Iranian output in September fell by 100,000 barrels per day, while production from OPEC as a whole rose by 90,000 bpd from August.
"Our oil analysts believe there is now a growing risk it (crude) could touch $100 per barrel," HSBC said in its fourth-quarter Global Economics outlook.
Many analysts say OPEC will struggle to cover a decline in exports from Iran. Britain's Barclays bank, however, said, "OPEC has ample spare capacity."
Soaring crude prices and weak emerging market currencies may erode economic growth.
"Softening demand growth and new supply should cool the bullish sentiment and push prices lower by the end of the year," Barclays said.
Reporting by Jessica Resnick-Ault Additional reporting by Henning Gloystein in Singapore and Ahmad Ghaddar in London; Editing by David Gregorio and Leslie Adler