By Eric Yep 
 

Crude-oil futures slumped in Asian trade Monday in line with a fall in wider commodities markets after Greece voted to reject the terms of its current bailout agreement in a weekend referendum.

After being stuck in a narrow price range for several weeks, oil has slipped into bearish territory on the back of developments in Greece, a stronger greenback, the Iranian nuclear talks and strong U.S. oil supply.

On the New York Mercantile Exchange, light, sweet crude futures for delivery in August traded at $54.96 a barrel at 0332 GMT, down $1.97 in the Globex electronic session. August Brent crude on London's ICE Futures exchange fell $0.58 to $59.74 a barrel.

Nymex crude lost 4.5% last week, its largest one-week net and percentage decline since mid-March, and has been down for three consecutive weeks. U.S. markets were closed on Friday. Brent crude lost 4.7% last week and has been down two of the past three weeks.

Over the weekend, Greek voters rejected creditors' demands in a referendum, raising concerns about its exit from the eurozone and sending financial markets into a tailspin on Monday.

"This is causing extreme bearishness in the market as prices remain low, breaking several supports which we had believed to be strong," Daniel Ang, an analyst at Singapore-based Phillip Futures, said. He said the unexpected increase in U.S. crude inventories last week is also widening the Brent-WTI spread, which is trading at $4.80 a barrel.

Oil markets are also wary of negotiations over Iran's nuclear program expected to conclude by Tuesday, as further delays would make it tougher for the Obama administration to finalize the deal.

Key elements of the deal between Iran and six world powers were falling into place Sunday, but U.S. Secretary of State John Kerry warned there were important sticking points that may yet scuttle the deal.

Iran expects to double its crude exports to 2.3 million barrels a day soon after sanctions are lifted and is pushing other members of the Organization of the Petroleum Exporting Countries to renew the cartel's quota system, Iran's deputy oil minister for planning and supervision, Mansour Moazami, said.

The return of Iranian oil to the market will be delayed and gradual, and not significant until well into 2016, Michael Wittner, head of oil research at Societe Generale said. "At that point, the market will be able to absorb Iranian volumes," he maintained.

Meanwhile, Saudi Arabia lowered the official selling price for its benchmark Arab Light crude to Asia in August, but raised the price for its customers in Europe.

Nymex reformulated gasoline blendstock for August--the benchmark gasoline contract--fell 409 points to $1.9934 a gallon, while August diesel traded at $1.7891, 508 points lower.

ICE gasoil for July changed hands at $551.00 a metric ton, down $4.00 from Friday's settlement.

--BenoƮt Faucon, Summer Said and Laurence Norman contributed to this article.

Write to Eric Yep at eric.yep@wsj.com