By Timothy Puko and Matthew Cowley

Oil prices fell toward three-month lows Monday on worries about China's stock markets, a new flood of Iranian oil and the debt saga in Greece.

Chinese stock markets have plunged in recent weeks, a pessimistic sign for demand in the world's second-largest oil consumer. A nuclear deal with Iran is also pushing forward, amid signs Iran wants to export even more crude that previously expected. And a "no" vote in Greece's referendum on Sunday has also pushed investors into the dollar, which typically pushes oil prices down.

The slate of concerns comes at just the moment when some feel oil has gone past a tipping point. Oil slid to a two-month low late last week as data showed U.S. producers had sent more drilling rigs to work for the first time since December, damping hopes they had started massive cutbacks that would reign in a world-wide oversupply.

"All signs point south for oil prices," Capital Economics said in its Monday note.

Light, sweet crude for August delivery continued its slide Monday, down $2.63, or 4.6%, to $54.30 a barrel on the New York Mercantile Exchange. Nymex is trading in an extended session that includes limited holiday trading from Friday. U.S. oil is at its lowest intraday price since April 15.

Brent, the global benchmark, fell $1.65, or 2.7%, to $58.67 a barrel on ICE Futures Europe. Brent hit its lowest intraday price since April 14.

Capital Economics lowered its year-end price forecast by more than 8%, it said in its note. That puts U.S. oil at $50 a barrel to end 2015 and Brent at $55.

Oil for two months had stabilized near $60, and its break away from that area suggests a continuing slide ahead, said Mark Waggoner, president of brokerage Excel Futures. It is likely to slip just below $50 a barrel, the last area it stabilized in back in February, he said. Iranian exports and the resilience of U.S. producers makes it increasingly unlikely that bulls will see the production declines they expected, Mr. Waggoner added.

"I still think production is going to increase," he said. "There's a whole gambit" of bad news for oil prices Monday morning.

The Chinese government over the weekend halted all new initial public offerings and the central bank is expected to help investors buy equities, according to The Wall Street Journal. The turmoil in Chinese stocks is yet another sign of the wrenching economic transformation that is under way in the Asian giant. For investors, the concern is that the Chinese government may struggle to contain the problems, broadly slowing growth and demand for oil along with it.

The sudden slump in Chinese stocks "is a huge cause for concern and as such can't be bullish for oil," said Tamas Vargas, an analyst at PVM brokerage in London.

Negotiations between Iran and the west in talks over Iran's nuclear ambitions are also due to be completed this week. The Wall Street Journal reported Iran wants to double oil exports to 2.3 million barrels a day if a deal is reached and sanctions are lifted.

The victory for the "no" vote in Greece's referendum on Sunday has also prolonged the uncertainty in global crude-oil markets. The WSJ Dollar Index is up 0.3% Monday, and, because oil is a dollar-denominated commodity, a stronger dollar often drags prices lower.

Bjarne Schieldrop, chief analyst for commodities at SEB Markets, said investors shouldn't see the Greek vote as a buying opportunity. It is outweighed by the possibilities of new Iranian supply on the world market, he said.

"News of an Iran deal may come tomorrow and a final resolution over Greece is likely to take longer," said Mr. Schieldrop. "Thus, it looks risky to buy the Greek selloff today as news of an Iran deal may arrive before a Greek resolution."

Gasoline futures fell 3.5% to $1.9636 a gallon. Diesel futures fell 4.5% to $1.7570 a gallon.

Write to Timothy Puko at tim.puko@wsj.com and Matthew Cowley at matthew.cowley@wsj.com

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