NEW YORK (Reuters) - Brent oil fell nearly 4 percent on Thursday after a preliminary pact between Iran and global powers on Tehran’s nuclear program, even as officials set further talks in June and analysts questioned when the OPEC member will be allowed to export more crude.

Iranian Foreign Minister Mohammad Javad Zarif (3rd R) arrives at the Beau Rivage Palace Hotel during an extended round of talks on Iran's nuclear programme, April 2, 2015, in Lausanne. REUTERS/Brendan Smialowski/Pool

Traders had been fixated on the talks held in Lausanne, Switzerland for over a week as Iran tried to agree with six world powers on concessions to its nuclear program to remove U.S.-led sanctions that have halved its oil exports.

The sanctions against Iran will come off under a “future comprehensive deal” to be agreed by June 30, after it complies with nuclear-related provisions, Iranian Foreign Minister Javad Zarif told a news conference.

“If nothing is going to be signed until June, something could go wrong between now and then,” said Phil Flynn, analyst at Price Futures Group in Chicago.

Bob McNally, an adviser to former U.S. president George Bush who heads energy research firm Rapidan Group, noted Iran will need much patience as the “sanctions are not likely to be lifted until late 2015 or early 2016, though we could see slippage beforehand.”

North Sea Brent crude futures, the more widely-used global benchmark for oil, settled down $2.15, or 3.8 percent, at $54.95 a barrel, almost $1 above the session low.

U.S. crude futures settled down 95 cents, or 2 percent, at $49.14 a barrel, after falling nearly $2 earlier.

“I think the market over reacted and is now sitting back a little to think there is a lot more work to be done,” said Dominick Chirichella, senior partner at the Energy Management Institute in New York.

FINAL NAIL IN OPEC COFFIN?

Under the preliminary deal, Iran would shut down more than two-thirds of its centrifuges producing uranium that could be used to build a bomb, dismantle a reactor that could produce plutonium and accept intrusive verification. Iran also needs to limit enrichment of uranium for 10 years.

Sanctions have cut Iran’s oil exports to about 1.1 million barrels per day from 2.5 million bpd in 2012. The OPEC nation is keeping about 30 million barrels of crude on a fleet of tankers ready to be shipped when allowed, into a market already flooded with supply.

John Kilduff, partner at New York energy hedge fund Again Capital, said since Iran was certain to export more oil at some point, it was time other members of OPEC led by Saudi Arabia considered cutting their production.

The selloff in oil, which began in June 2014, accelerated in November after the Saudis convinced the broader group within OPEC to stick to its output and defend market share. Brent crashed from 2014 peaks above $115 and U.S. crude tumbled from above $107.

Chirichella agreed with Kilduff. “Come June, the market will again be rocked by expectations of higher Iranian supply. If the Saudis stifle another production cut, that could be the final nail in OPEC’s coffin as some of its members break away to do their own thing to support the market.”