India's environment court has slammed the government over the capital's horrendous air pollution, which it said was "getting worse" every day, and ordered a string of measures to bring it down.

The National Green Tribunal directed all vehicles older than 15 years be taken off New Delhi roads, pollution checks undertaken for all state-run buses and air purifiers installed at the city's busy markets.

Environmentalists welcomed Thursday the decision, saying policy makers were failing to heed the "emergency" facing the city of 17 million people.

"We applaud the tribunal's urgency on Delhi's pollution which is reaching toxic levels," Chandra Bhushan, deputy director general of the Delhi-based Centre for Science and Environment.

"But the measures need to go further to deal with issues like the 1,000 new vehicles coming onto the roads each day," he told AFP.

"The government has a legal obligation to introduce the tribunal's measures," he added.

The tribunal hit out at Prime Minister Narendra Modi's government for failing to provide to the court any "substantive" action plans to tackle Delhi's smog-filled air despite requests.

"Nothing substantive has been suggested ... for providing and controlling air pollution in Delhi primarily resulting from vehicular pollution and burning of plastics and other materials in (the) open," the tribunal said in a ruling on Wednesday.

"It is undisputed and in fact unquestionable that the air pollution of (National Capital Territory) NCT, Delhi is getting worse with each passing day," it said.

The WHO said Delhi had the world's highest annual average concentration of small airborne particles known as PM2.5, following a study this year of 1,600 cities across the globe.

These extremely fine particles of less than 2.5 micrometres in diameter are linked with increased rates of chronic bronchitis, lung cancer and heart disease as they penetrate deep into the lungs and can pass into the bloodstream.

Delhi authorities have disputed the WHO's report and bristled at suggestions the capital was worse than Beijing, where thick smog has triggered public health warnings and concerns that are mostly absent in Delhi.

The tribunal, in a case filed by a lawyer and activist, directed authorities to crack down on burning rubbish in the open, construct cycle tracks and bypasses for heavy vehicles.

"It is a constitutional and statutory duty of all the authorities and ministries to provide clean air to the people to breathe," it said.

The small particles blighting the air of Delhi and other major developing cities around the world are often dust from construction sites, pollution from diesel engines or industrial emissions.

The Indian capital also suffers from atmospheric dust blown in from the deserts of the western state of Rajasthan, as well as pollution from open fires lit by the urban poor to keep warm in winter or to cook food.