An Indian court on Wednesday challenged the Delhi city government to prove that its restrictions on cars were improving air quality in the world's most polluted capital.

The Delhi high court said a trial ban, that allows most private cars to use the roads only on alternate days, was causing inconvenience to the public due to inadequate public transport.

With pollution levels still running at 10 times the international safe limits, it ordered the government to submit data for January 1-7 by Friday so the impact of the ban could be assessed.

The Delhi government launched the trial ban last Friday in an attempt to thin the toxic blanket of smog that has settled over the city.

"In these six days you must have collected data related to pollution level, we think it's sufficient for you," the bench of Judges G. Rohini and Jayant Nath said, according to the Press Trust of India news agency.

"You will have to think about the inconvenience caused to public at large," the judges  said, responding to a handful of petitions against the scheme.

Air quality, which traditionally worsens in winter as the cooler air traps pollutants and people start lighting fires, remains poor despite the trial restrictions which are due to last 15 days.

Environmentalists, however, say it would be unrealistic to expect a rapid improvement and attribute the smog to low winds and a fall in daytime temperatures.

The Delhi government defended the scheme, saying it had helped decongest the city's clogged roads and resulted in less pollution from cars.

"We have increased the frequency of the metro, increased the number of buses, people are carpooling... all of Delhi has come together to support this campaign," the city's transport minister Gopal Rai told news channels after the court order.

"Vehicular pollution, which is the most harmful of PM 2.5, has come down significantly in the interior part of Delhi," he said.

PM2.5 refers to microscopic particles that penetrate deep into the lungs and are particularly harmful to health.

India's state-run System of Air Quality Weather Forecasting and Research showed PM 2.5 concentrations at a "very unhealthy" 272 level on Wednesday and advised against going outside.

It recorded Delhi's PM 2.5 concentration hovering between "very poor" and "severe" levels since January 1, with Monday's reading spiking to 280, well above the World Health Organization safe limit of 25.

A 2014 WHO survey of more than 1,600 cities ranked Delhi as the most polluted, partly because of the 8.5 million vehicles on its roads.