WASHINGTON—The week after a judge stymied U.S. government efforts to curb potential risks to the financial system from giant insurers, the International Monetary Fund issued a new warning about rising global financial market hazards posed by the sector.

“Across advanced economies the contribution of life insurers to system risk has increased in recent years,” the IMF said in a report released Monday.

Though the report said danger posed by insurers “clearly remains below that of banks,” it urged global policy makers to take further steps to shore up the safety and soundness of industry giants.

But the difficulty facing regulators in pursuing such policies was evident when a U.S. District Judge last Wednesday issued a ruling blocking an attempt by the American government to impose such measures on one of the largest U.S. insurers, MetLife Inc.

A panel of American regulators had branded the firm a “systemically important financial institution,” a designation that required higher capital and much stricter oversight. MetLife sued to block the designation, and Judge Rosemary Collyer sided with the insurer, saying the government had failed to justify its policy.