At least two people died and more than 20 were injured following an earthquake on the southern Italian island of Ischia, a popular holiday destination off the coast of Naples.

Italy's geological institute INGV said the 4.0-magnitude seismic event took place just before 9pm (19:00 GMT) and had its epicentre at a depth of 5km, a few kilometres north of the island.

Demetrio Martino, deputy government prefect in Naples, told RAI public television that seven people were trapped under the rubble of a collapsed building.

Two were extracted alive, three children were still trapped but rescuers had established contact with them, but "we are worried" about the other two people, Martino said.

The three children being rescued are aged 17 months, four and six, the ANSA news agency said.

A total of 26 people reported injuries, including two who were seriously hurt but in non-life-threatening conditions, ANSA also said, quoting local police.

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According to several Italian media reports, one of the victims was an elderly woman killed by plaster falling off a church.

The quake "was really strong and really scary," Donatella Migliaccio, a local councillor, told DPA news agency. "I heard that [several] people have died," she added.

Ischia, an island with more than 64,000 residents that hosts many more people during the summer, lies about 55km southwest of Naples.

Questions raised over extent of damage

Pictures and footage published by Italian media and on social networks showed people in the streets and seriously damaged buildings. The quake caused an electrical blackout and the evacuation of a hospital.

Initial rescue efforts were constrained by limited resources on the island and authorities sent more aid with a special boat from Naples.

Ferries were also readied to take to the mainland anyone who wished to leave the island.

The INGV had initially classified the earthquake as a 3.6 with a 10km-deep epicentre, raising questions about how such a relatively weak seismic event could wreak so much havoc.

"It is not normal [for it] to cause building collapses and hospital evacuations," Egidio Grasso, the head of a regional association of geologists, said in a statement.

He suggested that the destructive effect may have been amplified by local geological factors or by shoddy constructions "built without any earthquake-proofing".

Ischia is famous for its thermal baths, and is a regular destination for German Chancellor Angela Merkel during her Easter holidays.

The island was struck by a 5.8-magnitude earthquake in 1883 that killed more than 2,300 people.

The latest disaster took place hours after Italian Prime Minister Paolo Gentiloni promised more reconstruction efforts following last year's earthquakes in central Italy, which killed more than 300

people.

Much of Italy's landmass and some of its surrounding waters are prone to seismic activity with the highest risk concentrated along its mountainous central spine.

Italy straddles the Eurasian and African tectonic plates, making it vulnerable to seismic activity when they move.