BEIJING—A U.S. blockade of Chinese-built islands in the South China Sea risks triggering a dangerous military confrontation and would be too costly to sustain long-term, experts warned Thursday after U.S. President-elect Donald Trump’s pick for secretary of state proposed such a move.

The proposal by Rex Tillerson in his Senate Foreign Relations Committee confirmation hearing Wednesday is likely to have alarmed Chinese leaders as it went well beyond Mr. Trump’s own remarks, as well as the advice from more hawkish elements...