An underwater volcano erupted just off Spain's El Hierro island in the Canaries, which has been rocked by thousands of small tremors since July, local officials said Monday.

The effects of the eruption, some five kilometres (three miles) from the island at a depth of more than 500 metres (1,500 feet), will not be felt on the island, said volcanologist Alicia Garcia of the Higher Council of Scientific Research.

"The earthquake monitoring stations on the island recorded the start of an eruption overnight. There is no visible manifestation at the moment," she told AFP.

The regional government of the archipelago issued a "yellow" volcanic eruption alert -- the second level on a scale of four -- after the island was hit by a 4.3-magnitude quake.

The earthquake was the strongest of more than 8,000 tremors which the Spanish National Geographic Institute has recorded on El Hierro since they began in July. The majority have been too small to be felt.

"There was a volcanic eruption overnight, what we don't know is what is its state, if it has just started and if it is just releasing gases or if it is releasing magma as well," the head of the island's local council, Alpidio Armas, told Spanish public radio.

"If the magma gets out, which appears will be the case, it will mean that the seismic movements will decrease," he added.

Armas said no further safety precautions were necessary on the island, but that a security perimeter marking the eruption area in the sea would be needed.

El Hierro, which means iron in Spanish, is the smallest of Spain's Canary Islands, located in the Atlantic off the coast of Morocco. It is home to some 11,000 people.

The last volcanic eruption on the Canary Islands was on nearby La Palma in 1971.