"The pace of urbanisation in the world has caught us all by surprise," Anna Tibaijuka said in Nairobi, citing that city's vast, 800,000-strong Kibera slum as a prime example.

"By 2030, 51 percent of Africans will be living in cities and towns. Africa will stop being a rural continent."

Unchecked flows of rural poor seeking better lives has put an unbearable strain on Africa's capitals, she said.

Some 70 percent of Nairobi's roughly three million inhabitants, for example, live in shanty-towns like Kibera.

"Urban poverty was not an easy issue to sell but people are catching on to its importance," she added at a news conference.

The solution lies not in forcibly stopping people from coming to cities but in making rural areas and smaller towns more attractive to live in with better services and commercial opportunities, she said.

The issue of Africa's urban poor has hit headlines in recent weeks with Zimbabwe's crackdown on shantytowns and informal traders leaving an estimated 200,000 people homeless.

AFRICA BEHIND UN TARGETS

UN Environment Programme head Klaus Toepfer, also at the news conference to discuss the UN Millennium Development Goals, agreed better services were the solution.

"If the electricity doesn't go to the people, the people will go to the electricity," he said.

Neither official mentioned Zimbabwe. Nor would they be drawn on questions from local journalists about forced evictions from the environmentally-crucial Mau forest in west Kenya.

One of the UN targets is to achieve "significant improvement" in the lives of at least 100 million slum-dwellers around the world by 2020.

Tibaijuka, a Tanzanian who sat on the Africa Commission set up by UK leader Tony Blair to prepare a blueprint for the upcoming summit of G8 rich nations, said the number of global slum-dwellers was soaring to an expected 1.36 billion by 2015.

That compares with 1 billion now and 700 million in 1990.

Toepfer gave a bleak outlook for sub-Saharan Africa in respect of the ambitious Millennium Goals, fixed in 2000 to eradicate poverty, improve education and health levels, promote gender equality and help the environment by 2015.

"Unluckily in sub-Saharan Africa we are lagging behind in nearly all those eight goals," said Toepfer, whose UNEP is based in Kenya. "So we have especially to hurry up in Africa."

Both UN officials urged G8 leaders to produce concrete results at their Gleneagles meeting, hosted by Blair who has put Africa and global warming as his priorities.