India's maiden test of an upgraded version of its nuclear-capable, medium-range Agni-II ballistic missile ended in failure Friday, when the missile dropped into the Bay of Bengal.

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.

"Shortly after initial take off, it deviated from its course and fell into the sea," an official with the Defence Research and Development Organisation said.

"The data and details are being studied to see exactly what went wrong" the official told AFP.

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.