By Stephanie Kelly

NEW YORK (Reuters) -Oil prices fell over 1% on Thursday, retreating from earlier gains, as investors weighed the potential effects of a U.S. court ruling that blocked the most sweeping of President Donald Trump's tariffs.

The market also watched for potential new U.S. sanctions curbing Russian crude flows and an OPEC+ decision on hiking output in July.

Brent crude futures settled down 75 cents, or 1.2%, to $64.15 a barrel. U.S. West Texas Intermediate crude fell 90 cents, or 1.5%, to $60.94 a barrel.

Prices had earlier risen after a U.S. court on Wednesday ruled that Trump overstepped his authority by imposing across-the-board duties on imports from U.S. trading partners. The court was not asked to address some industry-specific tariffs Trump has issued on automobiles, steel and aluminium using a different statute.

Futures steadily retreated throughout the session, however, as senior Trump administration officials downplayed the impact of the ruling and insisted there are other legal avenues to employ.

"The initial market reaction to the U.S. trade court's Trump reciprocal tariffs dissipated appreciably as the session progressed," Jim Ritterbusch of U.S. energy consultancy Ritterbusch and Associates said in a note.

"One interpretation of this response could be that not much has changed and that the uncertainty surrounding the Trump tariffs from day one will continue as the tariffs work their way through the court system and several sectoral tariffs such as autos and auto parts remain intact."

Weighing on oil futures on Thursday, IEA Executive Director Fatih Birol said in an interview with Bloomberg that demand for oil was considerably weak in China and developments in Russia and Iran were "question marks" for oil prices.

The U.S. and Iran are holding talks meant to rein in Iranian nuclear activities that have rapidly accelerated since Trump pulled Washington out of a 2015 deal between Iran and major powers that strictly limited those activities.

"We've seen a lot of back and forth concerns about the Iran situation, whether we’re getting closer to a conflict or a peace deal," said Phil Flynn, senior analyst with Price Futures Group. "We’re moving technically and emotionally right now in a lot of these markets."

OPEC+ IN FOCUS

On the oil supply front, the Organization of the Petroleum Exporting Countries and allies, together called OPEC+, could agree on Saturday to accelerate oil production hikes in July.

"We're assuming the group will agree on another large supply increase of 411,000 barrels per day. We expect similar increases through until the end of the third quarter, as the group increases its focus on defending market share," ING analysts said in a note.