HONG KONG—The U.S. declared its official opposition to a swath of Chinese claims in the South China Sea, in an unusually direct problem to Beijing’s efforts to assert regulate in the strategic waters.

Asserting the plan shift on Monday, Secretary of State Mike Pompeo characterised the decision as an energy to uphold global law from what he identified as a “might tends to make right” campaign by China to coerce and intimidate its Southeast Asian neighbors into ceding their passions.