Lt. Jaime Vallejo of Ecuador's Police Air unit said the students were reported missing Sunday while hiking up the Imbabura mountain.
A police helicopter did not find anything on its first flyover but continued searching, he said. Two rescue teams were also searching the mountain on foot.
One of the three missing Americans was identified by her family as Harvard University sophomore Jocelyn Karlan, 19.
Her mother, Dr. Beth Karlan -- a Los Angeles-based surgeon and professor at UCLA's David Geffen School of Medicine -- said her daughter and two missing friends were with about a dozen other students who set out to climb Imbabura on Sunday.
"They left early in the morning to climb up and the weather was quite inclement, with some rain, and it was chilly, and a bunch of the girls and guys turned back because of that, and she wanted to summit it," Karlan told The Associated Press by telephone Monday. She said she was notified her daughter was missing early Monday by friends who turned back.
Karlan said her daughter was an excellent athlete who played club soccer at Harvard. The missing students were not carrying much food or water and were last seen at 4:30 p.m. Sunday from a distance by another group of hikers, she said.
Imbabura, a 15,190-foot (4,630 meter) inactive volcano, is a prime tourist attraction some 40 miles (65 kilometers) northeast of Ecuador's capital, Quito.
U.S. Embassy officials were in contact with police and the families of the missing Americans, embassy spokesman Kevin Skillin said Monday.
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