Large parts of Indonesia, Malaysia and Singapore have been hit by smog from illegal bush fires burning on the islands of Sumatra and Borneo.
The haze has raised pollution in Singapore to near-record levels
Visibility in parts of Borneo was reduced to 50m while Singapore recorded its worst pollution levels since 1997.
Flights were cancelled, cars put their headlights on in the middle of the day, and Singapore warned citizens against taking exercise outdoors.
Indonesians use the fires for land clearance despite a government ban.
Hundreds of fires
Visibility was down to 50m in Central Kalimantan, on Indonesia's part of Borneo island.
Environmental agencies reported from 500 to 2,000 fires burning in the flammable peat soils of the region.
"The worst situation is in Central Kalimantan now. Most areas in the province contain peat," Malaysian forest fire chief Purwasto told Reuters news agency.
"We cannot estimate the extent of the fires now."
Singapore's environment agency recorded a pollution index level of 128 early on Saturday and said satellite data showed 506 fires burning on Sumatra island.
The pollution index is the highest level seen in Singapore since smog covered the region in 1997, causing billions of dollars in damage and lost tourist revenue.
The index had been at 80 on Friday. Any level above 100 is considered harmful.
Indonesia has outlawed using fire for land clearance but the laws are widely flouted in remote areas of the country, and the government seems helpless to control the situation, says the BBC's Lucy Williamson in Jakarta.
Pungent smoke from the fires is an annual problem across south-east Asia during the dry season.
Farmers have traditionally used brush fires in agriculture, but environmentalists claim the problem has become more serious in recent years due to timber and oil palm companies clearing land for plantations.
"The fires are seasonal and very predictable, but the government never implements effective measures to prevent and manage them," Nordin, a spokesman for Indonesian environmental group Save Our Borneo, told AFP news agency.