A staple vision of science fiction has moved closer to reality with the creation of a robot capable of making copies of itself.

The self-replicating machine, built in the United States, is extremely simple, able to perform no function other than duplicating itself and working only by assembling raw materials that have been prepared by human beings.

Even so, it offers the first proof of the principle that it is possible to design robots that can reproduce, repair themselves or even assemble components into completely new machines for use in environments where humans cannot work. One exciting application would be for exploration on planets such as Mars.

The concept of self-replicating robots and their potential benefits and risks have long been explored by science-fiction writers, who have often conjured up dystopian visions of machines that compete with humans for resources and survival.

Robots that can reproduce feature in the novels of Isaac Asimov and in films such as the Terminator series, often with disastrous consequences for humanity.

A variation on the theme was raised in Michael Crichton's novel Prey, which imagined self-replicating nano-robots drawing matter from living things and transforming the world into shapeless "grey goo."

The research, by a team at Cornell University in Ithaca, New York, does not threaten to make anything like this happen soon, but it does show that self-replication is not restricted to biological organisms.

The reproducing robots are made up of a series of cubes, known as "mole cubes," and copy themselves by picking up fresh cubes and assembling them into a new functioning machine.

Tshey are still the first robots to be built that can reproduce in this way. Details of the work were published in the journal Nature.

Hod Lipson, who led the research, said: "Although the machines we have created are still simple compared with biological self-reproduction, they demonstrate that mechanical self-reproduction is possible and not unique to biology."

The robots' component cubes contain electromagnets that allow them to attach to and detach from one another. Each cube is divided in half along a diagonal, which allows the robot to twist, bend, reconfigure itself and pick up other cubes from a "feeding" area.

"The four-module robot was able to construct a replica in 2.5 minutes by lifting and assembling cubes from the feeding locations," Doctor Lipson said.

"Because the replica is as large as the original, the replica reconfigures itself to assist in its own construction."