BUENOS AIRES, April 19 (Reuters) - A thick cloud of smoke covered Buenos Aires for a fifth day on Saturday, the fallout of field burning that has forced the closure of highways, flight delays and traffic congestion.

The smoke started to appear over the Argentine capital more than a week ago, but visibility deteriorated considerably in the city on Friday and Saturday, with an acrid smell pervading homes and causing watery eyes and sore throats among city residents.

Visibility downtown was barely 500 yards (metres).

Emergency services marshaled traffic in some areas of the city, while the capital's domestic airport Jorge Newbery diverted incoming aircraft to the international airport outside Buenos Aires -- where the smoke also caused some flight departure delays.

The dense smoke along highways in rural areas north of Buenos Aires caused traffic accidents that killed at least nine people, officials said on Friday.

Health officials sought to reassure the public that the smoke was not toxic, saying the material burnt was organic. However the municipality of Buenos Aires issued a "yellow alert" as a precaution.

Authorities said ranchers caused the haze by igniting fires across 70,000 hectares (173,000 acres) of pasture. The fires clear vegetation and renews soil nutrients and fresh pasture growth for their cattle.

Satellite images showed wind carrying a swathe of white over the capital and across the River Plate as far as Uruguay.

The government said on Friday efforts to contain the fires were failing and that it would prosecute farmers.

The national meteorological service said northerly winds would continue to carry the smoke toward the densely populated Buenos Aires area in coming days. It said there were no rains on the way to help extinguish the blaze.

Setting light to grasslands is common practice in Argentina, but the smoke cloud shrouding the capital is unusual. Officials put the phenomenon down to the extreme dryness of pastures coupled with a northerly wind, uncustomary at this time of year. (Reporting by Jorge Otaola, Writing by Simon Gardner, editing by Philip Barbara)

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