Three months after an international treaty banning cluster bombs came into force, stockpiles are already being destroyed by signatories to the convention, a monitoring group says.

"The US has been the most regular and most extensive user of the weapons, probably has the world's biggest stockpile with maybe as many as a billion sub-munitions in its current arsenal," said Stephen Goose, who edited the report and is director of the arms division at Human Rights Watch. (AP) More than 100 countries have agreed to ban the weapon, which scatters hundreds of smaller bombs over a wide area.

They are blamed for causing civilian casualties both at the point of impact and for years afterwards by remaining armed and in effect becoming landmines.

Some 108 states have signed the treaty.

The legally-binding agreement also holds countries liable for clearing areas where the bombs were dropped.

'Sea change'

In its first annual report, the Cluster Munition Monitor says seven countries have so far destroyed their supplies and two have cleared the areas where the bombs were dropped.

"The main message from the report is this is working - the movement to ban cluster bombs is working," said Thomas Nash, co-ordinator of the Cluster Munition Coalition.

The group of non-governmental organisations has lobbied hard to persuade countries to destroy their stockpiles of the weapons and to prevent their use and production.

The Convention on Cluster Munitions came into force in August and comes into full effect in the UK on Monday, which is now legally bound to clear places such as the Falkland Islands, where cluster bombs were used, within 10 years, and to destroy stockpiles within eight years.

Almost half the UK's cluster munitions have already been destroyed, the report said.

"Four years ago diplomats were saying it was impossible to get a ban on cluster bombs, but within four years we have brought about a complete sea change in government opinion around the world and brought about a new piece of international humanitarian law, the most important since the ban on landmines," added Mr Nash.

"The depository of the convention is the UN secretary general, but it was negotiated outside the traditional disarmament forum which was stale and unproductive, and I think that's been an important lesson. If things aren't working in a traditional mechanism we find another way."

The report says of the signatories, only 42 nations have so far ratified the treaty, and the US, which is thought to have the world's biggest stockpile of cluster bombs, has said it will keep them for a decade and has so far refused to sign.

"The United States has a somewhat untenable position on this issue," said Stephen Goose, who edited the report and is director of the arms division at Human Rights Watch.

"The US has been the most regular and most extensive user of the weapons, probably has the world's biggest stockpile with maybe as many as a billion sub-munitions in its current arsenal."

Cluster bombs were used in the first Gulf War, in Kosovo, Afghanistan, the invasion of Iraq, and possibly in Yemen in 2009, he said, but America had agreed in principle the weapon was damaging to civilians.

"They are in this untenable position of saying: 'We agree these weapons cause too much harm to civilians, but at the same time we want to use them for another 10 years'."

Laos is said to have been the most heavily bombed country in the world and is still the worst affected by the legacy of cluster bombs dropped during the Vietnam War.

The country will host an inaugural meeting of convention signatories and other nations next week to develop an action plan.