LONDON--The Organization of the Petroleum Exporting Countries boosted its closely watched forecast for oil demand growth for the second month in a row Wednesday, despite continuing to warn of possible emerging market headwinds.

In its monthly oil market report, the cartel of some of the world's biggest oil producers upgraded its forecast for demand growth this year by 50,000 barrels a day, after tweaking its expectations higher by the same amount last month. The producer group now expects oil demand to increase by 1.14 million barrels a day this year, largely as a result of higher consumption in North America, as well as improved demand in Europe and Africa. Total oil demand for 2014 is pegged 91.1 million barrels a day.

However "a key determinant for this increase in world oil demand will be the pace of growth in the emerging economies," OPEC said.

Concerns over slowing growth in emerging markets, particularly China, have hammered other commodities this week. Copper, often seen as a bellwether for the health of the global economy, slumped to its lowest level since July 2010, while iron ore prices fell to their lowest level since October 2012 on Monday.

The selloff in many base metals was triggered by growing concerns over the health of China's economy, following the country's first corporate bond default in its modern history, disappointing trade and manufacturing figures and weakness in the country's currency.

"If this continues then there will be more stories in the general media about the credit issues in China; what is currently happening in base metals could therefore start to spill into investors' exposure to oil commodities," oil consultancy Petromatrix said in a note Wednesday.

Oil prices were under pressure in London amid expectations of rising crude stocks in the U.S. and concerns over China. Brent crude for April delivery on London's ICE Futures exchange was down 69 cents, or 0.6%, at $107.86 a barrel. On the New York Mercantile Exchange, light, sweet crude futures for delivery in April were down $1.40, or 1.4%, at $98.63 a barrel.

However, while OPEC highlighted an economic slowdown in China and efforts to curb demand for transportation fuels in the country posed a risk to its current outlook, it argued that China's flourishing petrochemicals sector and expansions in refining capacity could be seen as the main factors driving the country's demand for oil this year.

Meanwhile, output from the group--which produces more than a third of the oil consumed globally each day--rose to its highest level since August last month as Iraq's production soared by 400,000 barrels a day, offsetting sharp declines from Saudi Arabia and Libya.

OPEC said that the changes to its demand outlook would mean an increase of 100,000 barrels a day in the demand for its oil, which it pegs at 29.7 million a barrels a day this year. At 30.1 million barrels a day in February, its production is comfortably above that level.

Write to Sarah Kent at sarah.kent@wsj.com

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