The Delhi government plans another round of tough measures to restrict the use of private cars and clean up toxic air in the Indian metropolis, the world's most polluted city.

From April 15, for two weeks, cars will only be allowed on the road on alternate days, going by whether their number plates are odd or even, Chief Minister Arvind Kejriwal said on Thursday.

"We are seriously considering if we can do this for 15 days every month," Kejriwal said. "We can't do this on a permanent basis until we get better public transport."

The rule hits most of the 2.6 million cars that ply the traffic-choked roads of Delhi and its surrounding areas. That figure accounts for roughly a third of all the 8.5 million vehicles on the streets, from motorbikes to autorickshaws.

Kejriwal said his plans had overwhelming public support and were necessary to rein in the rising levels of air pollution that regularly cloak the city in smog.

Doctors say Delhi's 16 million residents are at risk of suffering irreversible lung damage and some children there already have the lungs of chain smokers.

The new proposal follows a two-week trial at the beginning of January that took more than a million cars off the roads each day.

Most drivers followed the rules, with traffic sharply reduced from the usual rush-hour chaos.

The impact on pollution, though, was less clear, with data showing that air quality remained at unhealthy levels.

Following criticism over exemptions for women, politicians, judges, police and motorbikes, the government has said it is reviewing who will be exempt this time.

A survey showed about four-fifths of respondents supported the plans, with most respondents opposed to waivers for groups such as ministers, Kejriwal said.

"We will request VIPs to voluntarily follow the odd-even rule," he added. "Last time the chief justice and other judges followed it. Some of them even biked to work."

In a city known as India's rape capital, women will continue to be allowed to drive when they want if they are alone, he said.

The World Health Organization said in 2014 that New Delhi had the most polluted air among nearly 1,600 cities it studied.

A monitoring station at the United States embassy in the capital on Thursday recorded an air quality index of 157, showing that pollutants were three times the level deemed safe.

The US embassy recently added black to its colour-coded scale, to indicate AQI readings in excess of 500, a level off the scale used by the US Environmental Protection Agency.