An environmental group says diesel fumes are largely to blame for a dangerous increase in air pollution in Indian cities.

The Delhi based Centre for Science and Environment says diesel oil produced by public sector refineries in India is two hundred and fifty times dirtier than the world's best.

And it accuses the Indian authorities of encouraging the proliferation of diesel and two-stroke petrol engines, which cause the most damage.

The report says vehicle pollution in India is eight times higher than it was two decades ago, while pollution from factories has increased four-fold.