Agni-VI is an intercontinental ballistic missile

Intercontinental ballistic missile

An intercontinental ballistic missile is a ballistic missile with a long range typically designed for nuclear weapons delivery...

 being developed by India and the latest & most advanced version among the Agni (missile) program. Capable of launched from submarines or from land, it will be able to strike at a target of 6000km with MIRVed warheads.

The SLBM version of missile will arm the Arihant class submarines of the Indian Navy

Indian Navy

The Indian Navy is the naval branch of the armed forces of India. The President of India serves as the Commander-in-Chief of the Navy. The Chief of Naval Staff , usually a four-star officer in the rank of Admiral, commands the Navy...

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The program was classified and was never disclosed to public till 2011. However, in the DRDO newsletter in May 2011, the described achievements of a recently promoted scientist revealed that he headed a program codenamed A6; Which will be a 6000 km missile with underwater launch and MIRV capabilities. This only confirmed that India has been actively working on the Agni-VI ICBM. The mentions of A6 were soon removed and a modified version of newsletter was published on DRDO website. However, several bloggers and defense enthusiasts who had accessed the earlier version posted the excerpt on various defense forums.