Moving closer to its ambition of achieving the nuclear weapon triad - the capability to fire nuclear-capable missiles from the land, air and sea - India on Sunday successfully test-fired the underwater ballistic missile, K-15 (code-named B05), off the Visakhapatnam coast, marking an end to a series of developmental trials.

In its twelfth flight trial, the 10-metre tall submarine-launched ballistic missile (SLBM) lifted off from a pontoon, rose to an altitude of 20 km and reached a distance of about 700 km as it splashed down in the waters of the Bay of Bengal near the pre-designated target point.

According to scientific advisor to the defence minister V K Saraswat, the missile was tested for its full range of 700 km and the mission met all its objectives. He said the impact accuracy of the medium range strategic missile was in single digits.

Officials of the Defence Research and Development Organisation (DRDO) said the SLBM) was now ready for induction into the defence forces.

"The two-stage missile, which rapidly climbed to a 20-km altitude after being launched from the pontoon, was tested for its full range over 700-km. This was its last developmental test, in which all mission objectives were met," said an official.

"Now, the missile, which can carry a one-tonne nuclear warhead, will undergo a user-trial within a month before it's integrated into the submarine.

''Only the US, Russia, France and China have successfully developed SLBMs till now," he added.

The next major test is to test whether the K-15 missile can be launched from the 6,000-tonne INS Arihant, the country's first indigenous nuclear-powered submarine, which is now all set to undergo "sea-acceptance trials" after its miniature 83 MW pressurized light-water reactor goes "critical" at Visakhapatnam soon.

Built with four silos to carry 12 K-15s, or four of the 3,500-km range K-4 missiles under-development at present, INS Arihant will of course have to test-fire the 10-tonne missile during the sea trials before it can be said that India's long-awaited nuclear triad has finally become operational.

The first two legs of the triad, already in place with the armed forces, are the Agni series of missiles and fighters like Sukhoi-30MKIs and Mirage-2000s configured to deliver nuclear warheads.

The absence of an operational SLBM, however, has for long left a "big credibility gap" in the country's nuclear deterrence posture. Only a nuclear-powered submarine, which can stay underwater for extended periods, armed with nuclear-tipped missiles can provide a country with effective and difficult-to-detect "second or retaliatory strike capabilities".