"We cannot afford to abandon countries which sit in the 'too difficult' box," Benn told delegates on the eve of an international conference to begin developing a coherent approach to so-called fragile states.

"To prevent human disasters such as that in Darfur and to guard against global insecurity ... it is essential that we prevent countries becoming marginalised and excluded from the benefits of development and poverty reduction," he said.

"These are very difficult questions. But they have to be asked -- and answered," he later told Reuters. "We can't take sides. But the world is moving fitfully towards intervening in fragile states."

He strongly rejected any comparison with US President George W. Bush's doctrine of pre-emptive strikes against perceived failing states which could become enemies.

"This is not retaliatory aid," he said.

Jim Adams, vice president of the World Bank -- one of the conference's prime movers -- said his organisation had been one of the main culprits in walking away from difficult countries or setting them impossible targets. But times had changed.

"We have to learn to take more risks. It is a slow process. We have to lower expectations -- to be prepared for failure," he told Reuters.

Citing examples from Afghanistan to Zimbabwe, Benn said one-third of the people living in absolute poverty were to be found in fragile states -- countries where the rule of law was slipping or non-existent or where civil war was brewing.

These states accounted for 40 percent of all child mortality and their rates of HIV infection were four times those in other developing countries.

"We know that there can be no development without peace and security -- and that conversely, sustainable development is a precondition for long-term peace and security," he said.

Benn, who last month called for a complete overhaul of the UN's crisis relief system which he said had failed millions of people, said the world had to develop an early warning system to detect the imminent collapse of weak states.

Once detected, these states should become a focus for special attention to ensure that specialists remained in place to direct aid and support to the people who most needed it.

"We need to stay involved in fragile states even where the costs are high," Benn said, citing Indonesia's Aceh and northeast Sri Lanka -- devastated by the Asian tsunami but where decades of strife have destroyed trust and made aid distribution far harder.

Benn stressed that staying committed in fragile states demanded a rethink of the standard international aid formula that mostly means that money is withheld until peace prevails.