Italian Vice Premier Matteo Salvini initially said that eight people had been confirmed dead. The interior minister later said that no deaths had yet been confirmed, while 10 to 12 were missing.
The mayor of Naples was quoted by the news agency ANSA as saying one body had been recovered on the island.
Ischia sits in the Bay of Naples.
“The situation is very complicated and very serious because probably some of those people are under the mud,” Interior Minister Matteo Piantedosi told RAI state TV from an emergency command center in Rome.
ANSA reported that at least 10 buildings had collapsed. One family with a newborn that was previously reported missing had been located and was receiving medical care, according to the Naples prefect.
Video from the island showed small bulldozers clearing roads, while residents used hoses to try to get mud out of their homes.
Firefighters and the Coast Guard were working on rescue efforts. Reinforcements arrived by ferry, including teams of sniffer dogs to help the search efforts.
The densely populated mountainous island is a popular tourist destination for both its beaches and spas.
A 4.0-magnitude quake on the island in 2017 killed two people, and caused significant damage to the towns of Casamicciola and neighboring Lacco Ameno. - AP