Pollution levels reached a new record high 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 the country's history. The haze is also affecting Malaysia, with another 100 schools closed in the south of the country. Indonesia has prepared helicopters and cloud seeding equipment to try to tackle the fires. 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' A PSI reading above 300 is defined as "hazardous", while Singapore government guidelines say a PSI reading of above 400 over 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 back down to 245 at 14:00 (0600 GMT), although this is still in the "very unhealthy" range, and above peak levels during the 1997-1998 South East Asian Haze. 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%. "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. Diplomatic strain 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 in 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. 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. Reports say around 100 Indonesian firefighters are attempting to put out the fires in Sumatra. Indonesian President Susilo Bambang Yudhoyono said "all the country's resources" would be mobilised to extinguish the fires.