NEW DELHI: India has test-fired a submarine-launched ballistic missile (SLBM) with a "longer range" than that of the existing one of 750-km in the quest towards building a credible nuclear weapons triad.The new SLBM with a range over 2,000 km, tentatively dubbed K-4 , was tested from a submersible pontoon in the Bay of Bengal on Monday. The test came up for discussion in the annual DRDO directors' conference, attended by defence minister A K Antony and national security advisor Shivshankar Menon here on Tuesday, said sources.The new missile, part of the "K" series of underwater missiles being developed by DRDO, will have to be tested several times, first from pontoons and then finally from submarines, before it can become operational.While India for long has had land-based Agni missiles and fighters jury-rigged to carry nuclear weapons for deterrence, constituting the land and air legs of the triad, the lack of an operational SLBM has been a big operational gap.The 750-km range K-15 SLBM, which has undergone around a dozen tests from pontoons, is yet to be tested from a submarine. That will happen only after the first indigenous nuclear submarine INS Arihant goes for sea trials later this year.Though the miniature 83 MW pressurized light-water reactor on board the 6,000-tonne INS Arihant went "critical" on August 10 last year, it is yet to attain the full power needed for the submarine to head for sea trials.During these 18-month-long extensive "sea-acceptance trials", the 10-tonne K-15 missile -- which can carry a one-tonne nuclear payload – will be fired from the four silos on the submarine's hump. Two more nuclear submarines are being built to follow INS Arihant under the secretive ATV (advanced technology vessel) project at the ship building centre in the naval dockyard at Vizag.