Oil prices posted their largest slide in nearly two weeks on doubts that major producers can agree on supply curbs when they meet over the weekend.

Three straight days of losses are now foreshadowing what may become of the oil markets after a highly anticipated meeting Sunday between leaders of the Organization of the Petroleum Exporting Countries and non-OPEC producers, primarily Russia, analysts said. The possibility that the world’s biggest producers will start working together to boost prices has spurred oil’s sharpest rally in years, but many believe that a deal is unlikely and that oil prices will sell off again after the meeting.

Light, sweet crude for May delivery settled down $1.14, or 2.75%, to $40.36 a barrel on the New York Mercantile Exchange. For the week, the contract ended up 1.6% for its eighth winning week in the last nine.

Brent, the global benchmark, settled down 74 cents, or 1.7%, at $43.10 a barrel on ICE Futures Europe. It also ended the week higher by 2.8%.

Both benchmarks had risen overnight but dropped after news that Iran’s oil minister, Bijan Zanganeh, won’t attend the oil producers’ summit Sunday in Doha, Qatar.