India carried out an unsuccessful maiden test Friday of an upgraded version of its nuclear-capable, medium-range Agni-II ballistic missile. The surface-to-surface "Agni-II plus" was fired from a mobile rail launcher on Wheeler Island off the coast of the western state of Orissa, but dropped into the sea shortly into its planned flight, the sources said. It was not immediately clear why the test had failed. The missile has two stages, both fuelled by solid propellants. Capable of carrying a one-tonne conventional or nuclear warhead, Agni-II is one of a series of missiles being developed as a deterrent strategy against India's nuclear-armed neighbours China and Pakistan. It has a normal range of 2,000-2,500 kilometers (1,250-1,500 miles) and India has already successfully tested an Agni-III variant with a reach of up to 3,000-3,500 kilometers. The upgraded "Agni-II plus" has an improved navigation system and is aimed at bridging the gap between the two with a range of around 2,500-3,000 kilometers.