By Georgi Kantchev And Nicole Friedman 

Oil prices fell Tuesday as investors awaited the outcome of Iranian nuclear talks, which could pave the way for more Iranian crude to hit the already-oversupplied global market.

Tuesday is the deadline for Iran and six other nations to outline the main elements of a deal constraining Iran's nuclear program in exchange for lifting international sanctions. The deadline for a final agreement is the end of June.

Light, sweet crude for May delivery fell 68 cents, or 1.4%, to $48 a barrel on the New York Mercantile Exchange. Brent, the global benchmark, slid $1.12, or 2%, to $55.17 a barrel on ICE Futures Europe.

Many market participants worry that if sanctions are lifted, Iran, which holds around 10% of the world's oil reserves, would ramp up exports and add to the global glut of oil.

"A deal on Iran could depress prices in the short term by $3-5 a barrel and slow down the recovery in the second half of the year," said Giovanni Staunovo, analyst at UBS.

Oil prices have shed about half of their value since last summer because of a combination of oversupply and weaker demand. Analysts estimate that between 1 and 1.5 million barrels a day of excess crude oil is being produced.

Despite a price rebound in February, oil futures are set to end the first quarter of the year lower, which would mark the third quarterly decline in a row.

But some analysts caution that even if a deal on Iran's sanctions is reached, the impact on oil markets won't be immediate. Beyond Iranian crude-oil inventories stored on tankers, which are estimated to amount to 35 million barrels, "it is highly unlikely that Iran would be able to increase production until sometime in 2016," said Ed Morse of Citigroup in a note.

A strengthening dollar weighed on crude-oil prices Tuesday. Oil is priced in dollars, and a stronger dollar makes oil more expensive for buyers using foreign currencies.

Traders are watching the presidential elections in Nigeria. Votes are still being counted, and the outcome is likely to be contested. Nigeria produced between 1.8 and 1.9 million barrels a day of crude oil in February, according to the Organization of the Petroleum Exporting Countries.

Instability in the oil-producing Niger Delta region "poses a risk to the country's oil sector," wrote Eurasia Group in a note Tuesday.

Gasoline futures fell 2.5% to $1.7560 a gallon. Diesel futures fell 1.3% to $1.7095 a gallon.

Write to Georgi Kantchev at georgi.kantchev@wsj.com and Nicole Friedman at nicole.friedman@wsj.com

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