By Timothy Puko and Matthew Cowley 

Oil prices on Monday skidded to their biggest single-day declines in more than three months, as gyrations in Chinese stocks and the prospect of more crude from the U.S. and Iran revived worries about the global supply glut.

China's stock markets have plunged in recent weeks, which sparked worries among investors about oil demand in the world's second-largest consumer. Diplomats are trying to hammer out a final deal on Iran's nuclear program at the same time Iranian officials have signaled they want to export even more crude than traders had expected. And a "no" vote in Greece's referendum on Sunday has also bolstered the dollar, pushing down the prices of many commodities in the process.

These concerns are coming to the fore on the heels of data showing the first rise in the number of rigs drilling for oil since December. Also last week, U.S. weekly figures showed an increase in crude-oil stockpiles for the first time in nine weeks.

"All signs point south for oil prices," analysts at Capital Economics wrote in a note on Monday.

The U.S. benchmark oil price slid for the third trading session in a row, closing down $4.38, or 7.7%, to $52.53 a barrel on the New York Mercantile Exchange. Monday's losses are the biggest in percentage terms for a single session since February, though they also included declines during limited electronic trading on the Friday holiday. Nymex crude settled at its lowest level since April 13.

Brent crude, the global benchmark, recently traded down $3.85, or 6.4%, to $56.47 a barrel on ICE Futures Europe. Brent was on course for its biggest losses in one session and lowest settlement price since the week of April 6.

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.

For two months, oil prices had remained relatively stable at around $60 a barrel, and their breakaway from that area means the selloff is likely to deepen, said Mark Waggoner, president of brokerage Excel Futures.

Oil 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 make 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.

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.

In Vienna, Iran and six world powers are negotiating in an effort to reach a final agreement on curbing Iran's nuclear program. 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. Greeks overwhelmingly voted against their international creditors' conditions for further bailout aid, increasing uncertainty about Greece's future in the eurozone.

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.

"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."

In refined products, August gasoline ended down 11.06 cents, or 5.4%, at $1.9237 a gallon on the Nymex, while August diesel lost 13.10 cents, or 7.1%, to $1.7089 a gallon.

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

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