The avalanche struck a group of 16 mountaineers while at an altitude of 6,100 meters (20,000 feet) on the dormant Chimborazo volcano, according to responders.
"There are three missing mountaineers, three dead, three injured and seven rescued, out of a total of 16 people," Quito firefighters, who provided emergency support, said in a statement.
A previous report from the country's ECU911 security service had said four climbers had died and one was injured.
Authorities did not identify the climbers or their nationalities, but the Quito newspaper El Comercio reported that all were Ecuadoran.
The avalanche was spurred by "weather conditions" and was not caused by any volcanic activity, according to the firefighters.
Rescuers as well as police and military personnel specialized in high-elevation operations set up a command center at Chimborazo to coordinate search-and-rescue operations.
The volcano, which rises more than 6,200 meters high, is covered in snow and glaciers year-round and is located about 130 kilometers (80 miles) south of Quito.
Chimborazo attracts both Ecuadoran and foreign climbers. The towns of Riobamba and Ambato lie at its base.
Ecuador's Environment Ministry temporarily closed the surrounding Chimborazo nature reserve, which is popular with tourists. Despite the volcano's constant snow, it is not frequented by skiers.
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