By Alex Kirby BBC News Online environment correspondent in Johannesburg

National leaders assembling here for the World Summit on Sustainable Development have heard an impassioned appeal to think globally.

It came from a senior United Nations official, Jan Pronk, the special envoy to the summit of the UN secretary-general, Kofi Annan.

We must stop this trend in inward-looking values... if we fail to include everyone we will breed resentment, which may also breed violence

Jan Pronk, UN special envoy

The world could choose between an inclusive society and one that excluded the poor, he said. And exclusion could help to breed resentment and violence.

Mr Pronk said the world faced "a stark choice" between providing opportunities for the poor to develop, or continuing to let the rich get richer at the poor's expense.

There were even more people living below subsistence level than in 1992, at the time of the Rio Earth Summit, he said.

Unresolved issues

The last 10 years, Mr Pronk said, had seen slow environmental progress but "a stalemate" on poverty.

He said: "Post-September 11 last year, we have seen security being the overwhelming preoccupation of a country which is already safe.

Pronk: World faces a stark choice

"We have stoked up fears about aliens, strangers and 'illegals'. We can go for an exclusive society, with the poor and underdeveloped always excluded, or we can go for a world that is a safe place, where people have safe homes and jobs.

"We must stop this trend in inward-looking values, because if we fail to include everyone we will breed resentment, which may also breed violence."

Other UN officials say there are still 14 areas of serious disagreement to be resolved by delegates - not only ensuring clean water and energy for the world's poorest people, but also debt relief, development aid, globalisation, and the terms of trade.

They say two days of preliminary talks have managed to settle only 10 of 411 disputed issues.

One of the most divisive is whether the conference should end with binding commitments or aspirations.