A leaked Pentagon report confirms Russia has developed an underwater nuclear drone so destructive, it's known as a "doomsday" weapon.

This is the first time the US Defense Department has acknowledged the existence of the high-tech unmanned weapon, which is intended to destroy coastal targets such as the US's two nuclear missile submarine bases in Georgia and Washington state.

If deployed, the weapon's blast and radioactive fallout would also kill vast numbers of civilians.

A nuclear ballistic missile submarine enters St Petersburg in July 2017. Photo: Getty Images

Called Status-6, and nicknamed "Kanyon" by the Pentagon, the drone has a range of 10,000km and can carry a 100-megaton nuclear warhead.

The draft version of this year's Nuclear Posture Review was obtained by The Huffington Post .

"The Nuclear Posture Review has not been completed and will ultimately be reviewed and approved by the President and the Secretary of Defense. As a general practice, we do not discuss pre-decisional, draft copies of strategies and reviews," a Defense Department spokesperson told The Huffington Post .

Part of the leaked Nuclear Posture Review. Photo: supplied

The draft reveals that Russia has continued to produce nuclear weapons and develop technology at a rapid rate since the end of the Cold War in 1991.

It argues the US, on the other hand, has been reducing its weapons stockpile, and therefore needs to increase its capacity to counter that of Russia.

This is in line with reports last October that President Trump wanted to increase the US's nuclear capabilities.

The review argues in favour of investment in the full triad of nuclear weapons: air, sea and ground missiles.