Global economic slowdown could put a lid on oil prices but there is a risk that "nasty supply surprises" could reignite a market rally, the International Energy Agency said on Thursday.

The agency, which advises industrialized countries on energy policy, said market fundamentals had "clearly eased since the start of the year" and that oil stocks had built up significantly over the last few months.

But the IEA said any move by the Organization of the Petroleum Exporting Countries to adhere strictly to its declared production target of 30 million barrels per day (bpd) "would risk a potentially damaging price surge."

The IEA said the 12 members of OPEC pumped close to 31.8 million bpd in June, before the imposition of an EU oil embargo on Iran and tougher U.S. sanctions.

The agency kept its estimate of global oil demand growth steady at around 800,000 bpd for this year, rising to around 1 million bpd in 2013.