Twelve major cities including London, Paris, Los Angeles and Cape Town promised on Monday to buy only zero-emissions buses from 2025 and to make major areas free of fossil fuel emissions by 2030 to protect the environment. The 12, with a combined population of almost 80 million, said they would promote walking, cycling and the use of public transport under a joint "fossil-fuel-free streets declaration."

Many cities are setting tougher environmental goals than governments to limit air pollution and to achieve the goals of the 2015 Paris climate agreement to curb greenhouse gas emissions.

"Air pollution caused by petrol and diesel vehicles is killing millions of people in cities around the world," Paris Mayor Anne Hidalgo said in a statement. "The same emissions are also causing climate change."

The mayors, part of the C40 group of cities which is seeking to slow global warming, said they would "procure only zero-emissions buses from 2025 and ensure that major areas of their city are zero emissions by 2030."

Other cities signing up were Copenhagen, Barcelona, Quito, Vancouver, Mexico City, Milan, Seattle and Auckland.

Encouraging investments