The announcement by Secretary of State Mike Pompeo portends more American military operations to push back Chinese maritime activity and sanctions on Chinese companies.

Secretary of State Mike Pompeo denounced China on Monday, saying its maritime claims in the South China Sea were “completely unlawful.”

WASHINGTON — Secretary of State Mike Pompeo announced on Monday that China’s expansive maritime claims across most of the South China Sea were “completely unlawful,” setting up potential military confrontations with Beijing and sanctions against companies as the United States seeks to push back Chinese activity in the region.

Mr. Pompeo said China’s yearslong “campaign of bullying to control” offshore resources across much of the area was illegal. The announcement was the strongest and most explicit support by Washington of a ruling in 2016 by an international tribunal at The Hague that China had violated international law with its actions.

Mr. Pompeo’s announcement aligns U.S. policy directly with that ruling and puts Washington in a position to enforce the tribunal’s decision, even though China has rejected it. The statement is not explicit on U.S. military aid, but leaves open the possibility that the United States might come to the defense of nations like Vietnam, Malaysia and the Philippines if clashes erupt because of Chinese aggression. The United States has a mutual defense treaty with the Philippines.

“The statement is a full-throated endorsement of the tribunal’s ruling,” said M. Taylor Fravel, a political scientist at the Massachusetts Institute of Technology who studies China’s territorial disputes and its military.

But the United States is sticking to its policy of neutrality on competing claims to legitimate land features such as the Spratly Islands, he added.

China and five other governments have competing claims to land features in the South China Sea, and China has also come into conflict with Indonesia over Chinese activity in waters by that large archipelago nation. China delineates its claims to the South China Sea with what it calls “a nine-dashed line,” a boundary that encompasses an area the size of Mexico and demarcates almost the entire region.