The United States and the world “will not allow Beijing to treat the South China Sea as its maritime empire,” Secretary of State Mike Pompeo said Monday, prompting China to hit back, reflecting a ratcheting up of tensions between the two powers amid deteriorating relations.

“We are making clear: Beijing’s claims to offshore resources across most of the South China Sea are completely unlawful, as is its campaign of bullying to control them,” Pompeo said in a statement, referring to the contentious and potentially energy-rich stretch of water.

The U.S. has long opposed China’s expansive territorial…