Oil prices slipped below $91 a barrel Monday as markets grappled with Greece's debt crisis and the release of crude reserves by the International Energy Agency.

By early afternoon in Europe, benchmark oil for August delivery was down 61 cents to $90.55 a barrel in electronic trading on the New York Mercantile Exchange. Crude rose 14 cents to settle at $91.16 on Friday.

In London, Brent crude for August delivery was down $1.07 to $104.05 a barrel on the ICE Futures exchange.

While the dollar gained on the euro early Monday and briefly pushed the Nymex contract below $90, optimism that Greek lawmakers will approve a euro28 billion ($39.8 billion) austerity plan on Wednesday helped the European currency recover.

The euro was up to $1.4222 on Monday from $1.4188 late Friday. A weaker dollar makes crude and other commodities cheaper for investors holding other currencies and tends to lift prices.

German Finance Minister Wolfgang Schaeuble warned Sunday that Greece will not receive its next tranche of international aid unless the plan is approved.

Crude fell last week after the International Energy Agency said it will make 60 million barrels available over a 30-day period, half of which will come from the U.S. Strategic Petroleum Reserve. Some analysts said the price decline could be temporary.

"We doubt the world's energy supply will be solved by such a paltry sum," energy consultant The Schork Group said in a report. "This move might assuage the market this summer, but it is by no means a long-term fix."

Others, however, noted that since other crude exporters like Saudi Arabia were maintaining crude output levels, supplies were likely to rise.

"The countries in the Gulf region apparently do not want to restrict production and this should mean a marked oversupply on the oil market in the coming weeks, which should push prices down further," said a commodity report from Commerzbank in Frankfurt.

Analysts also said prices were under pressure because speculators, seeking safer destinations for their money, have begun to back away from bets that oil prices will rise.

In other Nymex trading in July contracts, heating oil slid 0.54 cent to $2.74 a gallon while gasoline fell 1.16 cents to $2.77 a gallon. Natural gas futures dropped 5.1 cents at $4.18 per 1,000 cubic feet.