India's environmental court slammed the Delhi government on Wednesday for failing to improve its notoriously toxic air, as the capital spent another day blanketed in grey smog.

The National Green Tribunal demanded authorities hold a crisis meeting to come up with a strategy to tackle the haze that has worsened across the city in recent days as winter cloud traps pollutants.

"What is the status of air pollution? All you can say is that there is no pollution... All stakeholders who are dealing with air pollution indicate that Delhi is highly polluting," the bench said in remarks directed at the city government.

"The level of PM2.5 and PM10, both are more than prescribed limits. We cannot permit such a state of affairs causing serious environmental pollution to prevail," the bench said, according to the Press Trust of India.

A senior official told AFP that a meeting of top environmental experts was under way late Wednesday following the court's demand.

Successive Delhi governments have faced flak for failing to clean up the city's filthy air, ranked as the worst in the world by the World Health Organization.

Courts have been pushing authorities to act, including ordering a toll tax on the thousands of diesel-guzzling trucks entering the city every night.

Smog levels soar in the winter when thousands of poor people light fires to keep warm.

But unlike Beijing, which also suffers from hazardous haze levels, New Delhi does not issue public health warnings.

The court's demand comes as global climate change talks continued on Wednesday in Paris. Indian Prime Minister Narendra Modi has told the summit that rich countries should not force the developing world to abandon fossil fuels completely.

A WHO study of 1,600 cities released last year showed Delhi had the world's highest annual average concentration of PM2.5 particles, less than 2.5 micrometres in diameter.

PM2.5 particles are linked to higher rates of chronic bronchitis, lung cancer and heart disease as they settle deep in the lungs and can pass into the bloodstream.

Delhi traffic policeman Ram Pranesh Singh described his job as like inhaling "slow poison".

"The sweepers dust the sides of the roads, the cars pollute and I am in the middle inhaling the mix from morning to night," he said as he manned a busy crossing near New Delhi's presidential palace on Wednesday.

"We are not given masks... it can be a pretty thankless job," the 48-year-old said.