India announced Saturday that its first indigenously-built nuclear submarine is ready for sea trials, a step before it becomes fully operational, and called it a "giant stride" for the nation.

India unveiled the 6,000-ton INS Arihant -- Destroyer of Enemies -- in 2009 as part of a project to built five such vessels which would be armed with nuclear-tipped missiles and torpedoes.

Prime Minister Manmohan Singh said he was "delighted to learn that the nuclear propulsion reactor on board INS Arihant, India's first indigenous nuclear powered submarine, has now achieved criticality".

Criticality refers to the point at which a nuclear reaction is self-sustaining.

Singh described the development as "a giant stride in the progress of our indigenous technological capabilities" and said he hoped to see the submarine commissioned soon.

Arihant is powered by an 85-megawatt nuclear reactor and can reach 44 kilometres an hour (24 knots), according to defence officials. It will carry a 95-member crew.

India earlier activated the atomic reactor on-board the INS Arihant, paving the way it to begin sea trials.

The Indian navy inducted a Russian-leased nuclear submarine into service in April 2012, joining China, France, the United States, Britain and Russia in the elite club of countries with nuclear-powered vessels.

Nuclear submarines can function underwater without needing to surface regularly to be recharged, unlike their conventional diesel-electric counterparts.

India is due to receive the first of six Franco-Spanish diesel-electric Scorpene submarines in 2015, part of a multi-billion dollar project to modernise its navy.

The submarine announcement came just two days before India was due to launch its first indigenous aircraft carrier "INS Vikrant".

The navy will launch its first indigenous aircraft carrier Monday at Kochi, making India only the fifth country in the world with the capability to build such vessels.

The 40,000-tonne aircraft carrier sets a new global standard in terms of size and complexity, the defence ministry says.

After final fitting of equipment and extensive trials, the aircraft carrier is due to join the navy by 2018.

It will ply the seas alongside the former Russian aircraft carrier "Admiral Gorshkov", now the "INS Vikramaditya", due to be delivered by year-end after a delay of more than four years.

India has been Russia's top arms customer for years, but relations have frayed over delays and cost-overruns.

India is spending tens of billions of dollars upgrading its mainly Soviet-era military hardware.