China and Russia must continue to implement international sanctions that limit oil exports to North Korea, the top U.S. diplomat warned at the United Nations on Thursday.

Secretary of State Mike Pompeo affirmed “in no uncertain terms” that North Korea has breached a cap on oil imports. China and Russia are the chief suppliers of petroleum products to North Korea, but they have rejected western assessments about the oil imports and reject U.S. charges of nonenforcement, even as American officials decry “ship-to-ship transfers” of oil to North Korean tankers on the high seas.