Smoke blanketing the Argentine capital Buenos Aires has thickened, prompting the authorities to close airports and major highways.
The smoke started a week ago, caused by fires on grassland outside the city that are being blamed on farmers clearing the land to graze cattle.
The government has said it will prosecute anyone it finds to be responsible for lighting the fires.
Health fears are growing as more people complain of sore throats and eyes.
Argentina's health ministry says the smoke is not toxic although it contains high levels of carbon monoxide, says the BBC's Daniel Schweimler in Buenos Aires.
The international airport was still operating but authorities closed the domestic airport, ports and several major roads as visibility declined in some areas to just a few metres.
Firefighters are battling to control the flames that have spread over thousands of hectares of scrubland.
Extremely dry pastures are providing ample fuel for the fires.
"The fires aren't coincidental and they haven't been spreading by themselves," said Justice Minister Anibal Fernandez.
Farmers have rejected the charge, however, saying it is designed to distract attention from a row over tax rises on farm exports.
Earlier this month, farmers suspended a strike which had led to shortages but gave the government until the end of the month to address their concerns.
Hospitals are reporting increasing complaints of respiratory problems and sore eyes, and some shops have run out of dust masks.
Residents are being advised to stay indoors and keep windows closed but the smell of burning has filled houses and offices anyway, says our correspondent.
Buenos Aires residents are hoping for a change in the wind direction to bring back the "good airs" which gave their city its name, our correspondent says.
However, meteorologists do not expect any change for a few more days.