Image caption Cluster bombs release smaller "bomblets" over a wide area

UN member states have rejected a US-backed plan to introduce new regulations on cluster bombs - munitions which break up into hundreds of smaller bomblets.

The plan would have eliminated all cluster munitions made before 1980.

But human rights groups argued that an international convention banning such bombs already exists and that the new protocol would dilute its provisions.

The US said that it was "deeply disappointed" by the decision.

"The protocol would have led to the immediate prohibition of many millions of cluster munitions [and] placed the remaining cluster munitions under a detailed set of restrictions and regulations," the US embassy in Geneva said in a statement.

First developed during World War II, cluster bombs contain a number of smaller bomblets designed to cover a large area and deter an advancing army.

A total of 111 UN member states have already signed up to the Oslo convention prohibiting the production, transfer, and use of cluster munitions. The US, Russia and China have not.

A senior US official said the bombs were a military necessity for when targets were spread over wide areas, and that using alternative armaments would cause more collateral damage and prolong conflicts, Reuters reports.

The outcome of Friday's meeting in Geneva was welcomed by human rights campaigners who say cluster bombs are indiscriminate weapons that can fail to explode on impact and lie dormant, often causing injury to civilian years after conflict has ended.

"How often do you see the US, Russia, China, India, Israel and Belarus push for something, and they don't get it? That has happened largely because of one powerful alliance driving the Oslo partnership," said Steve Goose of Human Rights Watch (HRW).

The BBC's Imogen Foulkes, in Geneva, says that though the proposal would have eliminated millions of ageing cluster munitions, even military allies of the US, like Britain, chose not to support it.

Many UN member states felt, she says, that getting rid of some cluster weapons while officially sanctioning others would set a dangerous precedent, and might even legitimise their use in the long-term.

The US move was also opposed by the International Committee of the Red Cross (ICRC) and the top UN officials for human rights, emergency relief and development.

How a cluster bomb works

1. The cluster bomb, in this case a CBU-87, is dropped from a plane and can fly about nine miles before releasing its load of about 200 bomblets.

2. The canister starts to spin and opens at an altitude between 1,000m and 100m, spraying the bomblets across a wide area.

3. Each bomblet is the size of a drink can and contains hundreds of metal pieces. When it explodes, it can cause deadly injuries up to 25m away.