The eruption at Chile's Cordon Caulle volcano diminished Wednesday, scientists reported, but they warned that ash likely would keep spewing out for at least several more days.
That will keep airplanes grounded in parts of Argentina and make it difficult to keep herds healthy in one of Chile's prime dairy regions.
The eruption, which began Saturday, has covered fields, rivers and lakes, blocked roads and forced the evacuation of thousands of people living nearby.
A huge ash plume has meandered across the width of Argentina, forcing airlines and travelers into a continuing state of on-again, off-again flights. By Wednesday evening, the ash had reached Buenos Aires again, hovering at high altitude where it could potentially harm jet engines.
Chilean Agriculture Minister Antonio Galilea said the ash has caused fish kills just north of the volcano by contaminating rivers and lakes, and livestock cannot drink the water.
Some roads and farms are covered by ash up to 29 inches (50 centimeters) deep. Other areas were spared because of prevailing winds.
Vicente Nunez, director of the National Emergency Office, said vulcanologists reported that "there has been a decrease in seismic activity" in the Cordon Caulle, but he cautioned that the decline "is not conclusive." He said the lessening of a volcano's eruption is a process that varies and takes time.
Evacuated residents were being allowed to return briefly to their properties to feed livestock and pets in the region 900 kilometers (560 miles) south of Chile's capital, Santiago.
Galilea said farmers have stored fodder to feed their animals and authorities are prepared to help them if they run out.
"The animals were hungry and we had to feed them from our reserves of grass," said Miguel Oyarzun, a rancher with 50 head of livestock in the GolGol River valley. His fields are covered with ash.
The Cardenal Samore pass linking Chile and Argentina has been closed since the weekend. Public Works Minister Hernan de Solminihac said it will take two days to clear away the ash and snow covering it.
About 4,000 people had been evacuated from more than 22 communities. They began fleeing as swarms of earthquakes Saturday heralded the eruption.
In neighboring Argentina, the winter resort of San Carlos de Bariloche and other nearby towns were clearing out from under the ash Wednesday, but the area's airport was among several airfields that remained closed.
International carriers were rerouting and rescheduling flights to deal with a backlog of cancellations, and keeping close watch on the plume to see which routes in and out of Argentina were safe enough to fly.
Aerolineas Argentinas and Austral, the Argentine state-owned international and domestic airlines, canceled all flights on Tuesday within Argentina as well as to and from other countries because of the threat of ash damaing engines. At least six international carriers also suspended flights between Buenos Aires and cities in the U.S., Europe and South America.