Singapore has been hit by the worst air pollution crisis in its history, while government agencies try to induce rain in an attempt to stop forest fires spreading a smoky haze across three south-east Asian countries.

The average Pollution Standard Index (PSI) – the measurement for air pollution – hit 401 at midday today, beating previous records of 371 and 321 on the previous two days. A reading between 101 and 200 is considered unhealthy. Anything over 300 is “hazardous”.

It is six days since the clouds of smoke first descended on Singapore, the result of fires in neighbouring Sumatra, Indonesia. Blaze season usually runs from June to September, when land is illegally cleared in this fashion for palm plantations.

Download the new Independent Premium app Sharing the full story, not just the headlines

Singapore's environment minister flew to Jakarta today to discuss measures to tackle the forest fires, and the Indonesian National Disaster Management Agency said it plans to use helicopters in a "water-bombing" operation, alongside more than 100 firefighters on the ground.

The agency added that planes would be sent over parts of Sumatra in the next few days in a "cloud-seeding" effort to try to induce rain chemically.

Singapore’s Prime Minister, Lee Hsien Loong, has told people to remain indoors as much as possible, and warned that the smog could last for weeks.

Staff at Singapore’s famous Fullerton Hotel point to the skyline where the nearby Swissotel, The Stamford, used to be visible. It all but disappeared from view on Wednesday.

The hotel is now handing its guests masks before they head out; its revolving door at the front has closed, a small precaution to keep the noxious air from wafting in. The management has also issued eye drops for its staff.

More than 100 companies across Singapore sent their staff home, opting not to wait for the government to decide to stop work.

Along the Boat Quay waterfront, some restaurants are reporting a 30 per cent drop in tourist traffic since the haze arrived last weekend. At the BQ Bar, however, bookings have held up. “It doesn’t affect people that much,” says Sue Ksumayu, assistant bar manager. “They can take off their masks when they’re drinking.”

The National Environment Agency (NEA) has advised residents, especially the elderly, the young and those with respiratory problems, to avoid prolonged spells outdoors and school closures are being considered. Hospitals are preparing for an increase in the number of cases of asthma, bronchitis and conjunctivitis and the Singapore Manpower Ministry published guidelines for construction workers.

“People should also stop work until the PSI drops back below 200,” said Professor Goh Lee Gan, from the College of Family Physicians Singapore (CFPS). “The PSI level is very unhealthy... this is the first time it’s got so bad.”

Malaysia, which has also been badly affected by the haze, has closed 200 schools and banned open burning in some areas. Singapore’s Minister for the Environment and Water Resources, Vivian Balakrishnan, said he would push for action ahead of a hastily convened meeting with Indonesia’s Foreign Ministry in Jakarta. “No country or corporation has the right to pollute the air at the expense of Singaporeans’ health and wellbeing,” he said in a Facebook post.

Tensions escalated when Agung Laksono, the minister co-ordinating Indonesia’s response, told reporters Singapore should stop “behaving like a child”. Indonesian officials have added to the tension by suggesting Singaporean companies may be partly responsible for the fires. Wilmar International Ltd, Golden Agri-Resources Ltd and First Resources Ltd, Singapore-based firms with palm oil concessions in Indonesia, defended their position, saying they used only mechanical means to clear the land.