A U.S. special-operations unit and a contingent of Marines have been secretly operating in Taiwan to train military forces there, U.S. officials said, part of efforts to shore up the island’s defenses as concern regarding potential Chinese aggression mounts.

About two dozen members of U.S. special-operations and support troops are conducting training for small units of Taiwan’s ground forces, the officials said. The U.S. Marines are working with local maritime forces on small-boat training. The American forces have been operating in Taiwan for at least a year, the officials said.

The U.S. special-operations deployment is a sign of concern within the Pentagon over Taiwan’s tactical capabilities in light of Beijing’s yearslong military buildup and recent threatening moves against the island.

Taiwan and U.S. officials have expressed alarm over nearly 150 flights near Taiwan in the past week by Chinese military aircraft. The Chinese aircraft have included J-16 jet fighters, H-6 strategic bombers and Y-8 submarine-spotting aircraft and have set a record for such sorties, according to the Taiwan government.... Read More: WSJ