LONDON-A rout in oil prices is likely to continue into the first half of 2015 unless global output is reduced, the International Energy Agency said Friday.

Crude prices have fallen around 30% since June to hover near four-year lows amid sluggish demand growth and a surplus of oil supply as a result of booming production in North America. Though the IEA expects demand growth to pick up throughout next year after falling to its weakest level in five years in 2014, it forecasts demand will fall steeply at the start of 2015 compared to the end of this year. Meanwhile, robust oil supply shows no sign of abating.

"Supply/demand balances suggest that the price rout has yet to run its course. Our supply and demand forecasts indicate that barring any new supply disruption, downward price pressures could build further in the first half of 2015," the IEA said in its closely watched monthly oil market report.

The sharp slide in prices has shifted focus to the meeting of the Organization of the Petroleum Exporting Countries scheduled for later this month. In the past, the group of 12 major oil producers has acted to balance the market, reining in output when prices fell. However, during the recent slide in prices OPEC has yet to indicate that it has plans to respond in a coordinated way.

According to the IEA, OPEC's output fell by 150,000 barrels a day in October as small reductions from several of the group's members offset higher output from Libya. However, overall production of 30.6 million barrels a day still remained well above the group's agreed output ceiling of 30 million barrels a day-the sixth month in a row that the group has over-produced, the IEA said.

If OPEC doesn't act to cut its output, the sustained decline in oil prices could put pressure on higher-cost producers elsewhere. Earlier this week, the IEA's chief economist Fatih Birol said that if oil prices remain around $80 a barrel, investment in U.S. shale projects will likely fall by 10% next year. However, lower investment won't necessarily translate into a quick decline in production as efficiency gains and cost-reductions should make up for it in the short term, the IEA said.

"Should prices continue to fall, however, medium‐ and long‐term production may fall as companies forego development of costly projects," it added.

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

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