NEW DELHI – India’s federal government on Wednesday urged Delhi and the surrounding northern states to immediately tackle dangerous levels of pollution in the national capital that forced schools to close and sparked criticism of authorities.

Thick smog swathed Delhi, where pollution readings in some places peaked at 500, the most severe level on the government’s air quality index measuring the number of poisonous particles.

“Every possible step required to tackle the situation has been already identified, and the need of the hour is to put them into action,” Environment Minister Harsh Vardhan said in a message on social network Twitter.

Using the hashtag “#DelhiSmog”, Vardhan urged the state authorities to rein in pollution, even if that meant deploying helicopters to spray jets of water across the capital.

Delhi has become a “gas chamber”, its Chief Minister Arvind Kejriwal said, as his government ordered schools closed until Sunday.

Anti-pollution measures adopted by the Delhi state government in recent years include limiting car use and taxing trucks that pass through the city, but few have succeeded.

Last November, the city struggled with its worst pollution in nearly 20 years, which forced about a million children to miss school, while thousands of workers reported sick and queues formed outside shops selling face masks.

Residents are worried this year as well. The Indian Medical Association has urged organisers to call off Delhi’s biggest race set for Nov. 19, to protect runners and volunteers.

If pollution increases, emergency measures, such as the suspension of construction activity, will kick in, the Delhi government has said.

The state government must adopt long-term anti-pollution measures, such as improving public transport and banning the use of dirty industry fuel, said Anumita Roychowdhury of New Delhi’s Centre for Science and Environment.

“Without the systemic changes, the scope and impact of the emergency measures will be limited,” she added.