The ''explosive remnants of war'' pact, clinched three years ago, has been ratified by more than the 20 states needed to become legally binding.
It requires the cleaning up of deadly debris such as unexploded shells, grenades, cluster submunitions, mortars and rockets which lie in wait after the end of hostilities.
''This is the first international agreement to require the parties to an armed conflict to clear all unexploded munitions that threaten civilians, peacekeepers and humanitarian workers once the fighting is over,'' the International Committee of the Red Cross (ICRC) said in a statement.
''Its entry into force, finally, is a milestone,'' Philip Spoerri, ICRC's director of international law, told Reuters.
The humanitarian agency -- hoping to minimise death, injury and suffering from Asia to Africa -- was behind the initiative to address unexploded ordnance in 2000.
Under the pact, warring parties must mark contaminated areas after a conflict ends and warn civilians of the risks until the ordnance has been cleared.
''It creates an obligation to clean up the mess on the battlefield even if a party doesn't control the territory anymore,'' Mark Hiznay, of the New York-based group Human Rights Watch, told Reuters.
El Salvador and Liberia, which have emerged from conflicts, are among 26 states to have ratified the treaty so far. U S President George W Bush has sent it to the Senate.
It is a protocol to the 1980 Convention on Certain Conventional Weapons (CCW) and its entry into force comes midway in a two-week review conference where momentum is building to address the issue of deadly cluster bombs, aid agencies say.
SENSE OF URGENCY Cluster bombs are air- or ground-launched canisters holding up to 650 submunitions, which often fail to explode on impact. Designed for use against infantry and tanks, they sink into the ground or lie on the surface and become virtual landmines.
Israel's use of cluster bombs in its month-long war against Islamist Hezbollah militia in southern Lebanon has brought a sense of urgency to halting their firing against military targets located in heavily populated areas, aid agencies say.
More than 20 people have been killed by cluster bomblets since the August 14 ceasefire in Lebanon, where experts estimate that an unusually high 40 per cent failed to explode on impact.
''A new treaty is needed urgently to prohibit cluster munitions, weapons that have caused documented and unacceptable harm for over 40 years. The devastation in Lebanon is just the latest example,'' Angelo Simonazzi, director-general of the Brussels-based Handicap International, said in a statement.
The ICRC has called for a ban on the use of cluster bombs in populated areas, where it says they cause ''severe and disproportionate impact'' on civilians, a call echoed by U N Secretary-General Kofi Annan.
Nearly 20 states at the Geneva meeting back an attempt by Sweden to seek the launch of negotiations on some form of treaty, according to the ICRC's Spoerri.
REUTERS AKJ BS1659