Diplomats from around the world are gathering in Dublin for a conference that aims to secure a treaty banning the use of cluster bombs.
The proposed ban has the support of more than 100 countries.
Humanitarian organisations say a binding treaty is now urgent because the weapons cause unacceptable harm to civilians.
But some of the world's main producers and stockpilers - including the US, the UK, Russia and China - oppose the move.See how a cluster bomb works
"Governments have been talking about the dangers of cluster bombs for years," said Grethe Ostern, joint head of the Cluster Munitions Coalition.
"More delays mean more injuries and death for ordinary people. We have a unique opportunity to ban cluster bombs in Dublin. It is now or never."
Cluster bombs have been used in countries including Cambodia, Kosovo, Afghanistan and Lebanon.
The initial weapon scatters thousands of smaller bombs across a wide area, but these bomblets can fail to explode, leaving a deadly legacy as civilians return to their homes.
On the eve of the conference, Pope Benedict XVI expressed hopes that "it will be possible to reach a strong and credible international agreement".
"It is necessary to heal the errors of the past and avoid them happening again in the future. I pray for the victims of the cluster munitions, for their families and for those who will join the conference too, wishing that it will be successful," the pontiff said.
Humanitarian groups, including the International Committee of the Red Cross, see the daily consequences of cluster munitions among the wounded civilians they treat.
They argue that a comprehensive and binding treaty - one that includes provision for compensation for victims - is essential.
"Cluster munitions are weapons that never stop killing," said ICRC president Jakob Kellenberger.
But some of the biggest producers and stockpilers of cluster weapons - the US, the UK, China, Russia, India and Israel - are against the ban, claiming such munitions can be useful on the battlefield.
They have been lobbying to have the treaty watered down.
While the UK government, for example, has shown a commitment to the process, it is strongly opposed to an outright ban.
Instead it is seeking an exemption for the newest generation of so-called "smart" cluster munitions which contain self-destruct mechanisms.
Campaigners say the failure rate of these new kinds of munitions makes them too risky.
If, as expected, the conference does come to agreement, it will be the most important disarmament treaty since the Ottawa convention to prohibit landmines over 10 years ago, the BBC's Imogen Foulkes says.
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