New Delhi: Indigenous nuclear submarine INS Arihant was commissioned into service in August, a report said on Tuesday.

By inducting the 6,000-tonne submarine into service, India has quietly completed its nuclear triad, reported The Hindu on Tuesday.

The daily quoted defence sources as saying that INS Arihant was formally commissioned by Navy Chief Admiral Sunil Lanba in August.

Sources added that in a bid to maintain secrecy, it is not being referred to as INS Arihant.

After being inducted, the submarine completes India's nuclear triad, giving it the capability to respond to nuclear strikes from sea, land and air-based systems.

INS Arihant gives second-strike capability to India, which has a clearly declared policy of "no first-use" of nuclear weapons.

INS Arihant is India's first indigenous nuclear submarine, and the lead ship of Arihant class of nuclear-powered ballistic missile submarines launched in 2009.

It was initially expected to go for sea trials by 2012, though this happened only in December 2014.

The vessel's miniaturised nuclear reactor, built with Russian help, went critical in 2013.

The project was undertaken by the Advanced Technology Vessel programme under the supervision of the Prime Minister's Office and involved agencies and establishments such as the Defence Research and Development Organisation, the Department of Atomic Energy and the Submarine Design Group of the Directorate of Naval Design, besides private companies such as Larsen & Toubro.

The submarine's design is based on the Russian Akula-1 class submarines and its 83 MW pressurised water reactor has been built with significant Russian assistance.

While its 100-member crew has been trained by Russian specialists, Indian scientists at the Bhabha Atomic Research Centre have received significant expertise in reducing the reactor size to help it fit into the 10-metre diameter hull of the submarine.

Nuclear submarines have the capability to stay out at sea longer, and don't need to surface for a long duration.

Conventional diesel-electric submarines have to come up to the surface at regular intervals for charging their batteries.