Fresh doubts are emerging about the potency of a U.S.-led sanctions campaign aimed at crippling North Korea’s economy and forcing the country to end its atomic-weapons programs, as denuclearization talks have stalled.

A confidential new United Nations report, reviewed by The Wall Street Journal, says Pyongyang, often with help from people in Russia and China, has been able to circumvent restrictions, rendering “the latest U.N. sanctions ineffective.”

Citing...