CAPE TOWN (AFP) – The world needs to act immediately to tackle an urban crisis of growing city slums that lack water and sanitation, a top UN official warned Tuesday on World Water Day.

"We have a crisis, we must recognise that," Joan Clos, UN Habitat executive director, told an event marking the day.

"We need to act now. This is a common action at the same time against the slums and against lack of water and sanitation. It's the same problem, it's the same solution: urban planning."

Targets set by the UN poverty-reduction Millennium Development Goals had seen improvements but unplanned urbanisation and unprecedented migration to cities have expanded slums without basic services.

"In general, more or less, everything is improving. But in the urban environment in some parts of the world and in Africa in particular we are facing a crisis. Things are not improving, they are worsening," said Clos.

According to the United Nations, 27 percent of the world's city dwellers in the developing world lives without piped water at home, while proper sanitation faces even greater challenges.

A UN report on Monday said 400 million of Africa's one billion people live in cities, a number projected to rise to over 1.2 billion by 2050.

Sixty percent live in slums, with those without safe drinking water at over 55 million and some 175 million without reasonable sanitation in 2008.