China has installed equipment on two of its fortified outposts in the Spratly Islands capable of jamming communications and radar systems, a significant step in its creeping militarization of the South China Sea, U.S. officials say. The move strengthens China’s ability to assert its extensive territorial claims and hinder U.S. military operations in a contested region that includes some of the world’s busiest shipping routes. The disclosure comes as the Chinese military is conducting what U.S. officials describe as its largest military exercise to date in the South China Sea, maneuvers that include China’s first aircraft carrier as well as...