SINGAPORE — Secretary of Defense Jim Mattis harshly criticized the Chinese government on Saturday for its continuing militarization of a string of islands in the South China Sea, calling the new presence of advanced military equipment and missiles there a flagrant show of military power.

“Despite China’s claims to the contrary, the placement of these weapons systems is tied directly to military use for the purposes of intimidation and coercion,” Mr. Mattis said during a speech on Saturday at the annual Shangri-La Dialogue, a regional security conference.

Mr. Mattis recently disinvited the Chinese military from a large, multinational naval exercise this summer due in part to China’s positioning of those weapons, including antiship and surface-to-air missiles, on the Spratly Islands.

China’s activities, Mr. Mattis said, are “in stark contrast to the openness our strategy promotes,” calling into question “China’s broader goals.”