Experts were keeping a close watch Thursday on a volcano in Spain's Canary Islands which last erupted 50 years ago after observing an upsurge in seismic activity and magma displacements.

This handout picture released on October 13, 2011 by the Spanish Government shows an aerial view of a green stain at sea off the coast of the village of La Restinga on the Spanish Canary Island of El Hierro. (Photo: AFP)

The earthquake swarm under La Cumbre Vieja on La Palma island began on Saturday and since then, there have been almost 1,000 tremors, the strongest with a magnitude of 3.4, the Involcan vulcanology institute said.

An earthquake swarm is a sequence of seismic events occurring in one place within a relatively short period of time.

"An intensification of the type of seismic activity in the coming days cannot be ruled out," it said.

The Spanish government's office in the Atlantic archipelago said the situation "could change quickly in the short term" but stressed there was "no clear evidence that suggested an eruption was imminent".

Cumbre Vieja is an active although dormant volcanic ridge in the south of La Palma that has erupted twice in the 20th century, first in 1949 then in 1971.

The authorities had on Tuesday raised the alert level from green to yellow, the second of four levels, in certain areas around the volcano, meaning civil protection officials must inform the public "to take precautions ahead of a possible volcanic eruption", the Pevolca emergency plan says.

Involcan said all of the quakes had been shallow, and a "significant ground deformation" as a result of "a small volume" of new magma flowing into the reservoir underneath the volcano, which amounted to 11 million cubic metres.

"Undoubtedly the current seismic swarm represents a significant change in the activity of the Cumbre Vieja volcano and is related to a process of magmatic intrusion beneath the island of La Palma," it said.