Illegal burning of forests and other land on Indonesia's Sumatra island to clear space for palm oil plantations is a chronic problem during the June to September dry season.
One Indonesian minister accused Singaporeans of acting like children, but pollution levels in the normally pristine city-state have shattered records set in 1997, raising diplomatic tensions and concerns about the economic impact.
"It can easily last for several weeks and quite possibly longer until the dry season ends in Sumatra," Lee told a news conference, warning of action if Singapore-linked companies were behind the burning. "On the scale of it, it's unlikely to be just small stakeholders slashing and burning."
On the fourth day of heavy smog, the smell of burned wood filled the air, skyscrapers were barely visible and haze hung in the tunnels that link Singapore's metro stations and shopping malls in the central core.
Some residents wore surgical masks or covered their faces with hankerchiefs when they walked outside. Singapore will suffer "an immediate hit to tourism", investment bank Barclays Plc said, noting that retailers, hotels, restaurants, gaming and other tourism-related sectors make up about 5-6% of the city-state's economy.
"We think arrivals will recover quickly when the haze dissipates," it said in a report. "But the situation is fluid -prolonged hazardous conditions could affect Singapore's international reputation."
An Australian couple on holiday said they cancelled a visit to the zoo and would probably stay indoors.
"I'm never coming back to Singapore at this time of the year again," said the husband, who identified himself only as Rob.
Singapore, a major financial centre, sent officials to an emergency haze meeting in the Indonesian capital, Jakarta.
"We will insist on definitive action," Environment and Water Resources Minister Vivian Balakrishnan said on his Facebook page.
"No country or corporation has the right to pollute the air at the expense of Singaporeans' health and well-being."
As Singapore put pressure on Jakarta to act, the Indonesian minister leading the response efforts hit back.
"Singapore shouldn't be like children, in such a tizzy," Coordinating Minister for People's Welfare Agung Laksono told reporters, adding the government was investigating whether large companies were behind the fires.
"If there are, some are owned by Indonesians, Malaysians, Singaporeans," he said. "We will take action if they are found responsible. But there must be a process."
To identify the culprits, Singapore has provided satellite data to Indonesia and will publish high-resolution photographs of the hotspots with the geographical coordinates to help interest groups such as Greenpeace pinpoint the sites.
Mcdonald's suspends deliveries
Singapore, which usually enjoys clear skies, saw its air quality deteriorate sharply on Monday.
Its pollution standards index soared to a record of 371 at 1 pm on Thursday and then swung between the "very unhealthy" and "hazardous" levels.
The pollution readings in Singapore have exceeded the peak of 226 hit in 1997 when smog from Indonesian fires disrupted shipping and air travel across Southeast Asia.
In Malaysia, the southern state of Johor was the worst affected. Air quality in the coastal town of Muar worsened in the "hazardous" category, forcing 211 schools to close.
Air traffic controllers in Singapore gave more time for aircraft between taking off and landing at Changi Airport, a major aviation hub, because of poor visibility.
"It's disgusting, terrible," Dennis Wong, an information technology professional at a foreign bank, said as he smoked a cigarette in the business district. "It feels very uncomfortable when you walk on the street. Better to stay at home."
Work at several Singapore construction sites slowed with few workers seen outdoors. Fast-food operator McDonald's suspended its delivery service across the city-state.
Singapore's drug stores have run short of face masks and residents have taken to social media to complain about their giant neighbour and the ineffectiveness of the Association of Southeast Asian Nations (ASEAN) to tackle the haze issue.
Indonesia's forestry minister, Zulkifli Hasan, said there were more than 100 hotspots - 80 percent of them on agricultural and plantation land and the rest in forests.
Singapore-based palm oil companies with land concessions in Indonesia include Wilmar International Ltd, Golden Agri-Resources Ltd and First Resources Ltd.
All three said on Wednesday they had "zero burning" policies and used only mechanical means to clear land. Cargill, whose Asia-Pacific hub is in Singapore, said there were no fires on its plantations in South Sumatra and West Kalimantan.
While companies may ban burning, those rules were hard for them and the central government to enforce with farmers and local officials, researcher Jackson Ewing told Reuters.
"The lack of actual control on the ground is a real issue," said Ewing, a fellow at the Centre for Non-Traditional Security Studies at Singapore's Nanyang Technological University, who has done studies at the plantations. The small-scale farmers, he said, were often "contracted by corporate entities".