The machine, created by a team at New York's Cornell University, bears more resemblance to a nursery toy than a Star Wars robot. It is built out of four smart building blocks, cannot walk or talk and still depends on a touch of human help.

But, its inventors say, it provides concrete proof that machines can be programmed to reproduce themselves, thus meeting a key conceptual requirement for building robots able to survive in an environment that is too harsh or remote for humans.

The four blocks are 10-centimetre (four-inch) cubes that have curved slits carved into the sides, enabling them to twist through an arc of 120 degrees.

The sides also have electromagnets that enable them to selectively attach and detach from each other. The modular robot can thus reconfigure itself into a tower, a right angle, a square, and so on.

And in each block is a small computer chip which is programmed with step-by-step instructions about what to do.

The material for the replication are blocks that are placed (by human hand) at a "feeding" point close by.

To start things off, the robot bends over and sets its top cube on a table. Then it bends to one side to pick up a cube from the feeding point and deposits that cube on top of the first.

By repeating the process, but swivelling and transferring cubes accordingly, one robot made up of a stack of blocks can create another just like itself.

And because one robot cannot reach across another of the same height, the robot being built helps to complete its own construction.

Self-replicating robots are standard fare in science fiction but extraordinarily difficult to achieve in practice, both in the challenges of technology and software.

So far only two machines that can reproduce have been unveiled, both of them primitive and nowhere near as successful as biological organisms.

The definition of self-replication is generally considered to be a machine that can build itself, and that this copy can then build another copy and so on.

Reproduction is a prerequisite in the biological world, but it also comes with the ability to adapt thanks to genetic changes in succeeding generations.

In a phone interview with AFP, lead researcher Hod Lipson said the robot, for all its limitations, was "proof of concept" that mechanical self-reproduction is possible and not confined to living organisms.

He and others have also been working on the evolutionary side of self-replication, fitting out a small lab robots with a computer and plastic-spraying inkjet printer that can design and make parts in order to meet to evolutionary challenges.

"One of our funding sources is NASA, which is very interested in the idea of self-repairing systems," he said.

"It's a growth area of interest -- high-performance robotics where you want systems that not only are able to diagnose a problem but can also repair themselves.

"If you sent a robot to Mars and something breaks, you want to be able to fix it. You don't want to lose your mission over something minor."

Self-replication and repair could also be used by robots that are sent into hostile environments, such as a war zone or a damaged nuclear reactor, where a human could not survive.

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