Media playback is unsupported on your device Media caption The BBC's David Shukman explains the impact and cause of the haze

Pollution levels soared for a third day in a row in Singapore, as smoky haze from fires in Indonesia shrouded the city state.

The Pollutant Standards Index (PSI) hit 401 at 12:00 on Friday (04:00 GMT) - the highest in Singapore's history.

The index also reached 400 in one part of Indonesia, which is readying helicopters and cloud-seeding equipment in an effort to tackle the fires.

Indonesia has said it is unfair to blame it solely for the forest fires.

A senior official in the Indonesian president's office said fires had been spotted on land owned by 32 companies in the region, some of them based in Malaysia and Singapore.

Schools in parts of Malaysia and Indonesia have closed temporarily.

Singaporean Prime Minister Lee Hsieng Loong warned on Thursday that the haze could remain in place for weeks.

"We can't tell how this problem is going to develop because it depends on the burning, it depends on the weather, it depends on the wind," he said.

"It can easily last for several weeks and quite possibly it could last longer until the dry season ends in Sumatra which may be September or October."

'Life threatening'

Analysis Indonesia is struggling to contain the raging forest fires that are causing the thick smog which is enveloping Singapore, parts of Malaysia and some Indonesian cities. On Friday, the government despatched helicopters to the worst affected areas, in a bid to create artificial rain. The plan is to seed the clouds once the temperature is a bit cooler to induce rain over the burning forestland. It is a big challenge. Fire-fighters on the ground have been working around the clock to put out the blazes, but they have spread to peatlands and are proving to be very difficult to extinguish. Officials have complained about a lack of resources and say they desperately need some rain to help. Indonesia's weather agency says rainfall is not likely until 29 June. Singapore and Malaysia have both urged Indonesia to do more to solve this crisis. Singapore has offered aircraft to help with the cloud-seeding operation, but there needs to be clouds in the sky for it to work. This time of year is typically the hottest and driest on the island of Sumatra.

A PSI reading above 300 is defined as "hazardous", while Singapore government guidelines say a PSI reading of above 400 sustained for 24 hours "may be life-threatening to ill and elderly persons".

"Healthy people [may also] experience adverse symptoms that affect normal activity," the government says.

The PSI dropped down to 143 at 17:00 (09:00 GMT), although this is still classed as "unhealthy".

Before this week's episode, the previous air pollution record was from September 1997 during the 1997-1998 South East Asian Haze, when the PSI peaked at 226.

Singapore resident Nicole Wu told the BBC that she had stayed indoors for the past two days.

"It's terrible. In my flat the windows are all closed with the air conditioning on," she said. "My mother has to wear a mask to go shopping."

"I can't even see what's happening outside my house due to the smog. You can't see birds [or] moving objects," she added.

Philip Koh, a doctor, told AFP news agency that the number of medical consultations he had had in the past week had increased by 20%.

Image caption Singaporeans have donned face masks as the haze engulfing the city hit hazardous levels.

Image caption On Friday the Pollutant Standards Index (PSI) hit 401, a level the government says "may be life threatening" to vulnerable people if sustained over 24 hours.

Image caption Demand for face masks has soared, leading many pharmacies to run out of stock.

Image caption The smog is caused by smoke from forest fires in Indonesia, which has blown over to southern Malaysia and Singapore.

Image caption Firefighters in Indonesia are said to be "overwhelmed and in a state of emergency".

"My patients are telling me they are worried about how long this is going to last and how much higher this is going to go," he said.

In Indonesia's Riau province, where the fires are concentrated, the PSI reached 400 on Friday, the head of the local health office told the BBC.

Schools there are to remain closed until the air quality improves.

The chief of the health department Zainal Arifin said there was an "increasing number of asthma, lung, eye and skin problems due to higher CO2 levels".

"I call for residents to stay at home and reduce outdoor activities," he said.

Diplomatic strain

The face masks which are in high demand in Singapore can protect against the worst of the smog... [but] are unlikely to provide total protection The health impact

Singapore's National Environment Agency has started providing hourly PSI updates on its website, in addition to the three-hourly updates it previously provided.

Around 300 schools in southern Malaysia have now been closed as a result of the smog. Schools in Singapore are currently closed for the holidays.

There are also reports of flight delays affecting both Singapore's Changi airport and Riau province in Indonesia.

The fires are caused by illegal slash-and-burn land clearance in Sumatra, to the west of Singapore.

The smog has strained diplomatic relations between Singapore and Indonesia - two countries that usually share good relations, the BBC's Karishma Vaswani in Jakarta reports.

Slash-and-burn clearances Slash-and-burn farming is a technique that involves cutting down vegetation and burning to clear land for cultivation

It is cheaper than using excavators and bulldozers

The illegal burning of forests to clear land for palm oil plantations has long been a problem in Indonesia - particularly during the dry season in the summer

Indonesia's Environment Minister Balthazar Kambuaya has said the government is investigating several palm oil companies in this respect

Some producers have already denied their companies use slash and burn land clearance

Mr Lee said Singapore had provided satellite date to Indonesia to help it identify companies involved and said that if any Singapore firms were involved, that would be addressed.

Indonesia's National Disaster Management Agency said it would deploy two helicopters to conduct "water-bombing" operations, as well as planes with cloud seeding equipment.

One of the Malaysian companies named by the Indonesian presidential official denied that it was burning forest to clear land, but said some small farmers operating on its property were doing so.

Palm oil giant Sime Darby said in a statement that it was strictly following its zero-burning policy throughout its operations, but that it could not control the activities of local growers farming on its concession area.

More than 100 Indonesian firefighters are attempting to put out the fires in Sumatra.

Media playback is unsupported on your device Media caption Selina Latiff, from Novena, Singapore, filmed this footage of the smog from a 29th floor balcony

However, an official in Riau province said they were "overwhelmed and in a state of emergency".

"We have been fighting fires 24 hours a day for two weeks," Ahmad Saerozi, the head of the natural resources conservation agency in Riau, told AFP news agency.

He added that the fires were in peat around three or four metres below the ground, making it particularly hard to fight them.

"It is still burning under the surface so we have to stick a hose into the peat to douse the fire," he said.

"We take one to two hours to clear a hectare, and by then another fire has started elsewhere."

Indonesian President Susilo Bambang Yudhoyono said "all the country's resources" would be mobilised to extinguish the fires.