U.S. oil prices slipped on Monday marked a modest decline, finding some support from crude export disruptions in Libya and recent data showing tighter U.S. supplies.

Brent prices, however, suffered a sharp loss as traders weighed expectations for higher global output. Crude had seen even sharper losses in early trading, stemming from a weekend tweet from President Donald Trump that talked about a big potential production increase from Saudi Arabia.

August West Texas Intermediate crude on the New York Mercantile Exchange US:CLQ8 fell by 21 cents, or 0.3%, to settle at $73.94 a barrel, after trading as low as $72.51. September Brent UK:LCOU8 dropped $1.93, or 2.4%, to $77.30 a barrel.

Last week’s surge to fresh 3 1/2-year highs for the U.S. benchmark, which contributed to strong monthly, quarterly and first-half 2018 gains, was somewhat disconcerting, said Barnabas Gan, an economist at Overseas-Chinese Bank. He added that the language in Trump’s weekend tweet—which suggested that Saudi Arabia may increase output by 2 million barrels a day—left scope for wide interpretation.

A senior Saudi Arabia official told The Wall Street Journal on Saturday that no specific promise had been made over production, but rather assurances were given that the country had the capacity to meet demand.

A White House statement released that same evening backed off that tweet from Trump, according to reports. It said King Salman of Saudi Arabia had told the president that his country would increase oil production “maybe up to 2,000,000 barrels.”

Saudi Arabia, OPEC’s swing producer, and Russia reached an agreement at a closely watched Vienna meeting last weekend to raise output less aggressively than had been anticipated.

Read: Investors look to the second half of 2018 expecting growth—amid rising uncertainties

Still, Saudi Arabia saw the biggest monthly jump in production since July of 2013—up 330,000 barrels a day in June to 10.3 million a day, according to a Bloomberg News survey. Overall OPEC output rose by 30,000 barrels a day to 31.83 million.

“The rise in supply from some OPEC and non-OPEC members will be met by a decline from others; doing the math here will be complicated for investors betting on the direction of prices,” said Hussein Sayed, chief market strategist at FXTM.

How Trump is tightening his squeeze on Iran

Meanwhile, Deputy Energy Secretary Dan Brouillette said Friday that countries that buy Iran’s oil would be given an opportunity to gradually cut back on their purchases, according to Bloomberg.

Brouillette’s comments were in contradiction to an earlier statement from the Trump administration that Washington would sanction countries that don’t cut oil imports from the Iran to “zero” by Nov. 4.

Experts anticipate the market will lose an additional 1. 5 million barrels of oil a day by the end of this year, primarily from outages in Venezuela and Iran.

And analysts are watching developments in Libya where major oil ports have been shut due to an internal power struggle in the country that has removed 850,000 barrels a day form the global oil market.

In a statement sent Monday to The Wall Street Journal, Libya’s National Oil Co. said it declared force majeure on the Eastern Libya ports of Zueitina and Hariga. A military faction controlling the area, the Libyan National Army, has been stopping its vessels from entering the ports.

Meanwhile, the U.S. supply picture showed signs of tightening Friday, after Baker Hughes reported that the number of U.S. rigs drilling for oil declined by four to 858 this week. It logged a second-weekly fall in a row.

Among other energy contracts, August gasoline US:RBQ8 shed 2.2% to $2.105 a gallon, while August heating oil US:HOQ8 lost 2.4% to $2.156 a gallon. August natural US:NGQ18 dropped 2.1% to $2.862 per million British thermal units.

— Neanda Salvaterra, Biman Mukherji and Benoit Faucon contributed to this article