MADRID — The Basque separatist group Eta signalled the start of a summer terror campaign yesterday when it detonated four bombs at tourist resorts and a branch of Barclays Bank in northern Spain.

No injuries were reported, but the co-ordinated attacks were seen by the Spanish authorities as a signal of intent during the holiday season.

The first device went off shortly after 5 a.m. outside a branch of Barclays in Neguri, near the Basque city of Bilbao, popular with British visitors.

The bomb shattered windows at the front of the building and blackened the entrance to the bank, damaging the cash dispenser machine. A caller claiming to representing Eta issued warnings of other devices planted in towns in the neighbouring province of Cantabria.

One exploded in the sand next to a beachfront promenade in the town of Laredo, near the ferry port of Santander, just after noon. About 40 minutes later, another detonated in sand dunes on the beach of Noja, about 20 miles from Laredo.

Police had cordoned off the towns after receiving the warnings. A fourth bomb then exploded on a golf course at Noja. Police were continuing to hunt for other devices last night.

The interior minister, Alfredo Perez Rubalcaba, said: “I want to condemn the attacks and issue a second message, just so it’s clear, that the best way to get a long prison sentence in Spain at the moment is to join Eta.”

Laredo and Noja are popular destinations in summer with tourists attracted to their long, sandy beaches facing the Bay of Biscay. Both towns are also popular with local day-trippers.

The explosions marked the beginning of Eta’s traditional summer bombing campaign aimed at disrupting tourism across the region. The blasts show that despite the organization being weakened by a large number of arrests in recent months it is not ready to abandon its four-decade struggle for an independent Basque state in northern Spain and south-western France.