OPEC is worried that its plan to drain a global oil glut—and thereby raise crude prices—isn’t working.

A long-planned meeting in St. Petersburg, Russia, on Monday to discuss the oil market with big producers outside the cartel has turned into a critical gathering. Over the weekend, the Organization of the Petroleum Exporting Countries said, its ministers have held a series of “intensive consultations” about the challenges for an output-cutting deal the 14-nation cartel struck last year with Russia and other big producers.

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