CARACAS, Venezuela—Venezuela’s oil output is collapsing at an accelerating pace, deepening an economic and humanitarian crisis and increasing the chances the country will default on its debts.

Crude oil production fell 12% in December from the month before, according to government figures released Thursday. Over all of 2017, output was down 29%, among the steepest national declines in recent history, driven by mismanagement and under investment at the state oil company, say industry observers and oilmen.

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