Oil Prices Fall to Lowest Level in 6 Months : Petroleum: Kuwait rejected OPEC limits and plans to boost production. Iran and Nigeria may follow suit.

NEW YORK — Oil prices plunged to their lowest level in almost six months Thursday after Kuwait rejected OPEC’s plan to keep production steady during the higher-demand summer months.

Although there were indications that the other members of the Organization of Petroleum Exporting Countries would largely abide by the production limits, some analysts expect Iran and Nigeria to follow Kuwait and increase production, depressing the price of oil further.

July-delivery of light sweet crude oil finished 36 cents lower at $19.28 per barrel, near the day’s low at the New York Mercantile Exchange. Trading was heavy once headlines began to emerge Thursday afternoon from the Geneva OPEC meeting.

“The bearishness comes in when you think about how will the other producers behave. That remains to be seen,” said Andrew Lebow, senior broker and analyst at E.D. & F. Man International Futures Inc.

Kuwait plans to increase its production by 500,000 barrels to match the 2.161-million barrel per day quota granted to the United Arab Emirates. Kuwaiti officials say they were promised a higher third-quarter quota when they agreed to a spring production cut.

Kuwait rejected on Thursday a higher OPEC quota of 1.76 million, which would have been slightly less than the Nigerian ceiling, the cartel said in a statement.

The 11 other members agreed to maintain spring production levels, which could help prices rise when demand increases in the Northern Hemisphere for summer driving.

OPEC has been trying, without much success, to force prices higher by restraining output. But some countries, more mindful of generating immediate income, have exceeded their quotas and undermined oil prices.

Peter Beutel, energy specialist at commodity brokerage Brody, White and Co. Inc., said he expects prices to fall as low as $18 per barrel if other OPEC countries disregard the output quotas and increase cartel production by about 1 million barrels to more than 2.5 million per day.

“When you see one person cheat, you get that twitchy upper lip you get when you feel out in the cold on something,” Beutel said.

The drop in crude prices and a continued heavy supply of gasoline in the United States helped unleaded gasoline for delivery in July drop 0.82 cent to 54.84 cents a gallon Thursday. Sluggish economies have also led some to believe that worldwide gasoline demand will be weak this summer.

Home heating oil for delivery next month settled at 53.17 cents a gallon, down 0.69 cent.

Natural gas prices were lower, with contracts for delivery next month settling at $2.141 per 1,000 cubic feet, down 3 cents.

In London, North Sea Brent Blend crude oil for delivery in July settled at $17.96 per barrel, down 30 cents, at the International Petroleum Exchange.