NEW DELHI, April 4 -- The Indian Navy on Friday commissioned its newest nuclear-powered ballistic missile submarine, INS Aridhaman, into service during a ceremony kept under wraps in Visakhapatnam as it was built under a highly classified programme to bolster the sea leg of the country's nuclear triad-the ability to launch strategic weapons from land, air and sea, top officials aware of the matter said.

It entered service on a day defence minister Rajnath Singh stressed that a powerful navy is "an absolute necessity."

Hours before the navy inducted the locally-built Aridhaman, Singh made a rare acknowledgement of the steady expansion of India's deployed nuclear forces. It is the country's third Arihant-class submarine, and will serve as...