NEW YORK—China’s ambassador to the U.S. warned Washington against “anti-China” alliances and a “Cold War mentality” as Beijing continued to ramp up its objections to U.S. military activity in the South China Sea in advance of a weekend meeting of defense chiefs from the U.S. and Asia.

In an interview here with The Wall Street Journal, Ambassador Cui Tiankai defended China’s recent expansion of reefs it claims in the Spratly Islands to include civilian navigation and some military facilities, which has spurred objections from...